diff --git "a/venv/lib/python3.10/site-packages/transformers/models/mega/modeling_mega.py" "b/venv/lib/python3.10/site-packages/transformers/models/mega/modeling_mega.py"
new file mode 100644--- /dev/null
+++ "b/venv/lib/python3.10/site-packages/transformers/models/mega/modeling_mega.py"
@@ -0,0 +1,2273 @@
+# coding=utf-8
+# Copyright 2023 The Mega Authors and The HuggingFace Inc. team.
+#
+# Licensed under the Apache License, Version 2.0 (the "License");
+# you may not use this file except in compliance with the License.
+# You may obtain a copy of the License at
+#
+#     http://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.
+"""PyTorch MEGA model."""
+
+import math
+from typing import List, Optional, Tuple, Union
+
+import torch
+import torch.nn.functional as F
+import torch.utils.checkpoint
+from torch import nn
+from torch.nn import BCEWithLogitsLoss, CrossEntropyLoss, MSELoss
+
+from ...activations import ACT2FN
+from ...modeling_outputs import (
+    BaseModelOutputWithPoolingAndCrossAttentions,
+    CausalLMOutputWithCrossAttentions,
+    MaskedLMOutput,
+    MultipleChoiceModelOutput,
+    QuestionAnsweringModelOutput,
+    SequenceClassifierOutput,
+    TokenClassifierOutput,
+)
+from ...modeling_utils import PreTrainedModel
+from ...pytorch_utils import ALL_LAYERNORM_LAYERS
+from ...utils import (
+    add_code_sample_docstrings,
+    add_start_docstrings,
+    add_start_docstrings_to_model_forward,
+    logging,
+    replace_return_docstrings,
+)
+from .configuration_mega import MegaConfig
+
+
+logger = logging.get_logger(__name__)
+
+_CHECKPOINT_FOR_DOC = "mnaylor/mega-base-wikitext"
+_CONFIG_FOR_DOC = "MegaConfig"
+
+
+from ..deprecated._archive_maps import MEGA_PRETRAINED_MODEL_ARCHIVE_LIST  # noqa: F401, E402
+
+
+class MegaEmbeddings(nn.Module):
+    """
+    Mega's basic implementation does not incorporate token type embeddings, so this is a stripped-down version of
+    RoBERTa's embeddings which optionally includes token types
+    """
+
+    def __init__(self, config: MegaConfig):
+        super().__init__()
+        self.word_embeddings = nn.Embedding(config.vocab_size, config.hidden_size, padding_idx=config.pad_token_id)
+        self.use_token_types = config.add_token_type_embeddings
+        if self.use_token_types:
+            self.token_type_embeddings = nn.Embedding(config.type_vocab_size, config.hidden_size)
+            # registering a buffer here allows model tracing when not passing optional token type IDs
+            # more info at transformers issue #5664
+            self.register_buffer(
+                "token_type_ids", torch.zeros(config.max_positions, dtype=torch.long).expand((1, -1)), persistent=False
+            )
+
+        self.padding_idx = config.pad_token_id
+
+    def forward(self, input_ids=None, token_type_ids=None, inputs_embeds=None):
+        if (input_ids is None) and (inputs_embeds is None):
+            raise ValueError("Must provide one of input_ids or inputs_embeds")
+        elif input_ids is not None:
+            input_shape = input_ids.size()
+            device = input_ids.device
+
+            # get the word embeddings if only IDs are provided
+            inputs_embeds = self.word_embeddings(input_ids)
+        else:
+            input_shape = inputs_embeds.size()[:-1]
+            device = inputs_embeds.device
+
+        # the original Mega implementation did not include token type embeddings, so we add
+        # an option to use them if desired; if embeddings are present and token type IDs are
+        # not provided, we will use a registered buffer (which helps with tracing)
+        if self.use_token_types:
+            if token_type_ids is None:
+                if hasattr(self, "token_type_ids"):
+                    buffered_token_type_ids = self.token_type_ids[:, : input_shape[1]]
+                    buffered_token_type_ids_expanded = buffered_token_type_ids.expand(input_shape[0], input_shape[1])
+                    token_type_ids = buffered_token_type_ids_expanded
+                else:
+                    token_type_ids = torch.zeros(input_shape, dtype=torch.long, device=device)
+
+            # access token type embeddings
+            token_type_embeddings = self.token_type_embeddings(token_type_ids)
+            # add the token type embeddings to the word embeddings
+            embeddings = inputs_embeds + token_type_embeddings
+        else:
+            embeddings = inputs_embeds
+        return embeddings
+
+
+class MegaSimpleRelativePositionalBias(nn.Module):
+    """
+    Simple relative positional embeddings copied from the Mega repo; renamed variables for better readability
+    """
+
+    def __init__(self, config: MegaConfig):
+        super().__init__()
+        self.config = config
+        self.max_positions = self.config.max_positions if self.config.chunk_size < 0 else self.config.chunk_size
+        self.rel_pos_bias = nn.Parameter(torch.Tensor(2 * config.max_positions - 1))
+
+    def forward(self, seq_len):
+        if seq_len > self.max_positions:
+            raise ValueError("Sequence length {} going beyond max length {}".format(seq_len, self.max_positions))
+
+        # seq_len * 2 - 1
+        bias = self.rel_pos_bias[(self.max_positions - seq_len) : (self.max_positions + seq_len - 1)]
+        # seq_len * 3 - 1
+        tile = F.pad(bias, (0, seq_len))
+        # (seq_len * 3 - 1) * seq_len
+        tile = torch.tile(tile, (seq_len,))
+        tile = tile[:-seq_len]
+        # seq_len x (3 * seq_len - 2)
+        tile = tile.view(seq_len, 3 * seq_len - 2)
+        start = (2 * seq_len - 1) // 2
+        end = tile.size(1) - start
+        tile = tile[:, start:end]
+        return tile
+
+
+class MegaRotaryRelativePositionalBias(nn.Module):
+    """
+    Rotary relative bias for positional information; similar in concept to RoPE (i.e. RoFormer) but taken from the Mega
+    repo due to differences in implementation.
+
+    When initialized, produces a positional bias which ranges from position 0 to config.max_positions, but can
+    extrapolate to longer sequences. Can be indexed according to input position IDs
+    """
+
+    def __init__(self, config: MegaConfig):
+        super().__init__()
+        if config.hidden_size % 2 != 0:
+            raise RuntimeError("Rotary positional bias requires `hidden_size` to be a multiple of 2")
+        self.config = config
+        self.embed_dim = config.shared_representation_size
+        self.max_positions = self.config.max_positions if self.config.chunk_size < 0 else self.config.chunk_size
+        self.sine, self.cosine = MegaRotaryRelativePositionalBias.get_sinusoid_embeddings(
+            config.max_positions, self.embed_dim
+        )
+        # alpha and beta parameters for the rotary bias; beta renamed to b_param to avoid clashes with tf/flax weight handling
+        # in loading pretrained weights
+        self.alpha = nn.Parameter(torch.Tensor(1, self.embed_dim))
+        self.b_param = nn.Parameter(torch.Tensor(1, self.embed_dim))
+        self.register_buffer("_float_tensor", torch.FloatTensor([0.0]))
+
+    @staticmethod
+    def get_sinusoid_embeddings(max_positions: int, embedding_dim: int):
+        half_dim = embedding_dim // 2
+        emb = math.log(10000) / half_dim
+        emb = torch.exp(torch.arange(half_dim, dtype=torch.int64).float() * -emb)
+        emb = torch.arange(max_positions, dtype=torch.float).unsqueeze(1) * emb.unsqueeze(0)
+        return torch.sin(emb), torch.cos(emb)
+
+    def rotary(self, input):
+        seq_len, embed_dim = input.size()
+        chunk_1, chunk_2 = torch.chunk(input, 2, dim=-1)
+        if self.sine is None or seq_len > self.sine.size(0):
+            self.sine, self.cosine = MegaRotaryRelativePositionalBias.get_sinusoid_embeddings(seq_len, embed_dim)
+            self.max_positions = seq_len
+        self.sine = self.sine.to(self._float_tensor)
+        self.cosine = self.cosine.to(self._float_tensor)
+
+        sin = self.sine[:seq_len]
+        cos = self.cosine[:seq_len]
+        return torch.cat([chunk_1 * cos - chunk_2 * sin, chunk_2 * cos + chunk_1 * sin], dim=1)
+
+    def forward(self, seq_len):
+        rotary_alpha = self.rotary(self.alpha.expand(seq_len, self.embed_dim))
+        rotary_beta = self.rotary(self.b_param.expand(seq_len, self.embed_dim))
+        bias = torch.einsum("mk,nk->mn", rotary_alpha, rotary_beta)
+        return bias
+
+
+class MegaDropout(nn.Module):
+    """
+    A unified class for standard dropout functionality and featurewise dropout.
+
+    The original fairseq Mega repo used 2 classes for these, which included some unnecessary handling of training logic
+    and an unused `inplace` option. The original implementation used torch.nn.functional instead of submodules, which
+    is retained here as well.
+    """
+
+    def __init__(self, dropout_probability, is_featurewise=False):
+        super().__init__()
+        self.dropout_probability = dropout_probability
+        self.is_featurewise = is_featurewise
+
+    def forward(self, input, batch_first: bool = False):
+        if self.is_featurewise:
+            if batch_first:
+                # (batch_size X sequence_length X feature_dimension)
+                # -> (batch_size X feature_dimension X sequence_length)
+                # -> (batch_size X sequence_length X feature_dimension)
+                return F.dropout2d(
+                    input.transpose(-1, -2), p=self.dropout_probability, training=self.training
+                ).transpose(-1, -2)
+            else:
+                if input.dim() != 3:
+                    raise ValueError(
+                        "Feature dropout inputs must be exactly 3-dimensional if inputs are ordered [sequence length, batch size, hidden dimension]"
+                    )
+                # (sequence_length X batch_size X feature_dimension)
+                # -> (batch_size X feature_dimension X sequence_length)
+                # -> (sequence_length X batch_size X feature_dimension)
+                return F.dropout2d(input.permute(1, 2, 0), p=self.dropout_probability, training=self.training).permute(
+                    2, 0, 1
+                )
+        else:
+            return F.dropout(input, p=self.dropout_probability, training=self.training)
+
+
+class MegaRMSNorm(nn.Module):
+    """
+    RMSNorm used in Mega implementation. Differs from T5's RMSNorm by applying the weight prior to taking the square
+    root (as opposed to after in T5)
+    """
+
+    def __init__(self, number_features, eps=1e-6, affine=True):
+        super().__init__()
+        self.num_features = number_features
+        self.eps = eps
+        self.affine = affine
+        if affine:
+            self.weight = nn.Parameter(torch.Tensor(self.num_features))
+        else:
+            self.register_parameter("weight", None)
+
+    def forward(self, input):
+        mean_square = torch.mean(torch.square(input), dim=-1, keepdim=True)
+        if self.weight is not None:
+            input = input * self.weight
+
+        input * torch.rsqrt(mean_square + self.eps)
+        return input
+
+
+class MegaScaleNorm(nn.Module):
+    """
+    Scale normalization introduced in MEGA which is similar to RMSNorm, but uses a single parameter for scalar
+    multiplication instead of a vector, and applies over a specified dimension
+    """
+
+    def __init__(self, dim, eps=1e-6, affine=True):
+        super().__init__()
+        self.dim = dim
+        self.eps = eps
+        self.affine = affine
+        if affine:
+            self.scalar = nn.Parameter(torch.Tensor(1))
+        else:
+            self.register_parameter("scalar", None)
+
+    def forward(self, input):
+        mean_square = torch.mean(torch.square(input), dim=self.dim, keepdim=True)
+        if self.scalar is not None:
+            input = self.scalar * input
+
+        output = input * torch.rsqrt(mean_square + self.eps)
+        return output
+
+
+class MegaSequenceNorm(nn.Module):
+    """
+    A wrapper class for various layer normalization options used in Mega. Used to handle differences in expectations on
+    input axis locations for different normalization methods.
+    """
+
+    def __init__(self, norm_type, embedding_dim, eps=1e-5, affine=True, export=False):
+        super().__init__()
+        if norm_type == "layernorm":
+            self.norm = nn.LayerNorm(embedding_dim, eps, elementwise_affine=affine)
+        elif norm_type == "scalenorm":
+            self.norm = MegaScaleNorm(dim=-1, eps=eps, affine=affine)
+        elif norm_type == "rmsnorm":
+            self.norm = MegaRMSNorm(embedding_dim, eps=eps, affine=affine)
+        elif norm_type == "batchnorm":
+            self.norm = nn.BatchNorm1d(embedding_dim, eps=eps, affine=affine)
+        elif norm_type == "syncbatchnorm":
+            self.norm = nn.SyncBatchNorm(embedding_dim, eps=eps, affine=affine)
+        else:
+            raise ValueError("Unknown norm type: {}".format(norm_type))
+
+    def forward(self, input):
+        if isinstance(self.norm, nn.modules.batchnorm._BatchNorm):
+            if input.dim() != 3:
+                raise ValueError("BatchNorm inputs must be exactly 3-dimensional")
+            input = input.permute(1, 2, 0)
+            input = self.norm(input)
+            return input.permute(2, 0, 1)
+        else:
+            return self.norm(input)
+
+
+# add this layernorm class to ALL_LAYERNORM_LAYERS
+ALL_LAYERNORM_LAYERS.append(MegaSequenceNorm)
+
+
+class MegaMultiDimensionDampedEma(nn.Module):
+    """
+    Mega's Exponential Moving Average layer, largely left unmodified from the original repo with the exception of
+    variable names and moving away from the stateful representation of incremental decoding state. See
+    "https://arxiv.org/abs/2209.10655" for more details.
+    """
+
+    def __init__(self, config: MegaConfig):
+        super().__init__()
+
+        self.config = config
+
+        self.embed_dim = config.hidden_size
+        self.ndim = config.ema_projection_size
+        self.bidirectional = config.bidirectional
+        self.truncation = config.truncation
+        self.scale = math.sqrt(1.0 / self.ndim)
+
+        kernel_dim = 2 * config.hidden_size if self.bidirectional else config.hidden_size
+        # renamed delta (damping_factor) and alpha (decay_factor) to be more descriptive of what the parameters are doing
+        self.damping_factor = nn.Parameter(torch.Tensor(kernel_dim, self.ndim, 1))
+        self.decay_factor = nn.Parameter(torch.Tensor(kernel_dim, self.ndim, 1))
+        # renamed gamma (kernel_projection_matrix) and beta (ema_expansion_matrix) respectively to avoid HF renaming
+        # things and align with the paper's description of these params' behavior
+        self.ema_expansion_matrix = nn.Parameter(torch.Tensor(kernel_dim, self.ndim, 1))
+        self.kernel_projection_matrix = nn.Parameter(torch.Tensor(kernel_dim, self.ndim))
+        # renamed omega to residual_weight to describe what it's doing
+        self.residual_weight = nn.Parameter(torch.Tensor(config.hidden_size))
+        self._kernel = None
+        self._coeffs = None
+
+    def _compute_ema_coefficients(self):
+        self._coeffs = None
+        # convert the alpha and delta parameters (kernel_dim x EMA projection size x 1) to [0, 1] with sigmoid
+        damping_factor = torch.sigmoid(self.damping_factor)
+        decay_factor = torch.sigmoid(self.decay_factor)
+        previous_timestep_weight = 1.0 - damping_factor * decay_factor
+        return damping_factor, previous_timestep_weight
+
+    def _compute_efficient_ema_kernel(self, length: int):
+        # computes the kernel used for efficient damped EMA applied via FFT convolution
+        self._kernel = None
+        # p and q have shape (kernel_dim x ema_projection_size x 1)
+        damping_factor, previous_timestep_weight = self._compute_ema_coefficients()
+        # extend the kernel to (kernel_dim X ema_projection_size X sequence_length) and
+        # multiply q by sequential ints up to the sequence length
+        vander = torch.arange(length).to(damping_factor).view(1, 1, length) * torch.log(previous_timestep_weight)
+        kernel = (damping_factor * self.ema_expansion_matrix) * torch.exp(vander)
+        # (kernel_dim X ema_projection_size X sequence_length) -> (kernel_dim, sequence_length)
+        return torch.einsum("dnl,dn->dl", kernel, self.kernel_projection_matrix * self.scale)
+
+    def get_ema_coefficients(self):
+        if self.training:
+            return self._compute_ema_coefficients()
+        else:
+            if self._coeffs is None:
+                self._coeffs = self._compute_ema_coefficients()
+            return self._coeffs
+
+    def get_ema_kernel(self, length: int):
+        kernel_size = length if self.truncation is None else min(self.truncation, length)
+        if self.training:
+            return self._compute_efficient_ema_kernel(kernel_size)
+        else:
+            if self._kernel is None or self._kernel.size(-1) < kernel_size:
+                self._kernel = self._compute_efficient_ema_kernel(kernel_size)
+            return self._kernel[..., :kernel_size]
+
+    def fft_convolution(self, inputs, kernel, length):
+        # this is a wrapper for repeated use of EMA calculation via FFT (fast Fourier transform) convolution
+        inputs_fft = torch.fft.rfft(inputs.float(), n=2 * length)
+        kernel_fft = torch.fft.rfft(kernel.float(), n=2 * length)
+        convolved_sequence = torch.fft.irfft(inputs_fft * kernel_fft, n=2 * length)
+        return convolved_sequence
+
+    def ema_step(self, inputs, length, past_state=None):
+        if length == 1:
+            return self.one_ema_step(inputs, past_state=past_state)
+
+        # (kernel_dim X ema_projection_size X 1)
+        damping_factor, previous_timestep_weight = self.get_ema_coefficients()
+        # (kernel_dim X ema_projection_size X 1+sequence_length)
+        vander = torch.arange(length + 1).to(damping_factor).view(1, 1, length + 1) * torch.log(
+            previous_timestep_weight
+        )
+        vander = torch.exp(vander)
+        if past_state is not None:
+            # (kernel_dim X ema_projection_size X sequence_length) * (kernel_dim X ema_projection_size X 1)
+            # -> (kernel_dim X ema_projection_size X sequence_length)
+            past_ema_proj = vander[:, :, 1:] * (self.kernel_projection_matrix * self.scale).unsqueeze(-1)
+            # past_state will be (batch_size, kernel_dim, ema_projection_size)
+            past_ema_state = torch.einsum("bdn,dnl->bdl", past_state, past_ema_proj)
+            # (kernel_dim X ema_projection_size) * (batch_size X kernel_dim X ema_projection_size)
+            # -> (batch_size X kernel_dim X ema_projection_size)
+            past_vandermonde = vander[:, :, -1] * past_state
+        else:
+            past_ema_state = None
+            past_vandermonde = None
+
+        # (kernel_dim X ema_projection_size X sequence_length)
+        vander = vander[:, :, :-1]
+        kernel = (damping_factor * self.ema_expansion_matrix) * vander
+        kernel_proj = torch.einsum("dnl,dn->dl", kernel, self.kernel_projection_matrix * self.scale)
+
+        ema_output = self.fft_convolution(inputs, kernel_proj, length=length)[..., 0:length]
+        ema_output = ema_output.type_as(inputs)
+        if past_ema_state is not None:
+            ema_output = ema_output + past_ema_state
+
+        updated_hidden_state = torch.einsum("bdl,dnl->bdn", inputs, torch.flip(kernel, dims=[2]))
+        if past_vandermonde is not None:
+            updated_hidden_state = updated_hidden_state + past_vandermonde
+        # return a tuple:
+        # (sequence_length, batch_size, kernel_dim)
+        # (batch_size, kernel_dim, ema_projection_size)
+        return ema_output.permute(2, 0, 1), updated_hidden_state
+
+    def one_ema_step(self, inputs, past_state=None):
+        damping_factor, previous_timestep_weight = self.get_ema_coefficients()
+        # (kernel_dim X ema_projection_size) x (batch_size X kernel_dim X 1)
+        # -> (batch_size X kernel_dim X ema_projection_size)
+        updated_state = (damping_factor * self.ema_expansion_matrix).squeeze(-1) * inputs
+        if past_state is not None:
+            updated_state = updated_state + previous_timestep_weight.squeeze(-1) * past_state
+        # (batch_size X kernel_dim)
+        out = torch.einsum("bdn,dn->bd", updated_state, self.kernel_projection_matrix * self.scale)
+        # (1 X batch_size X kernel_dim), (batch_size X kernel_dim X ema_projection_size)
+        return out.unsqueeze(0), updated_state
+
+    def forward(
+        self,
+        inputs,
+        attention_mask: Optional[torch.Tensor] = None,
+        prev_state: Optional[torch.Tensor] = None,
+        use_cache: bool = False,
+    ) -> torch.Tensor:
+        """
+        Mega's exponential moving average (EMA) sub-layer applied prior to single-headed (traditional) self-attention
+
+        Args:
+            inputs (`torch.Tensor` of shape `(sequence_length, batch_size, hidden_size)`):
+                Hidden state / embedding input to update via EMA based on FFT convolution
+            attention_mask (`torch.LongTensor` of shape `(batch_size, sequence_length)`, *optional*):
+                Indicates which inputs are to be ignored (mostly due to padding), where elements are either 1 for *not
+                masked* or 0 for *masked*
+            prev_state (`torch.Tensor` of shape `(batch_size, config.ndim)`, *optional*):
+                The hidden state returned from the previous timestep during incremental decoding.
+            use_cache (`bool`, default `False`):
+                Whether to perfom incremental decoding; uses `prev_state` as the prior timestep, and returns the
+                updated EMA hidden state for use in the next step
+
+        Returns:
+            `tuple(torch.FloatTensor)` containing various elements depending on configuration ([`MegaConfig`]) and
+            inputs:
+            - **hidden_states** (`torch.FloatTensor` of shape `(sequence_length, batch_size, hidden_size)`) -- Hidden
+              states updated by EMA, with same shapes as inputs
+            - **updated_state** (*optional*, returned when `use_cache=True`) `torch.FloatTensor of shape `(batch_size,
+              config.ndim)` -- The incremental EMA state for use in the next step of incremental decoding
+        """
+
+        seq_len, bsz, embed_dim = inputs.size()
+        if embed_dim != self.embed_dim:
+            raise ValueError(
+                f"Unexpected embedding dimension received: input is {embed_dim}, model expects {self.embed_dim}"
+            )
+
+        # sequence_length X batch_size X hidden_size
+        residual = inputs * self.residual_weight
+
+        # (sequence_length x batch_size x hidden_size) -> (batch_size x hidden_size x sequence_length)
+        inputs = inputs.permute(1, 2, 0)
+        # mask the input: output is a tensor with 0 in the masked positions
+        if attention_mask is not None:
+            inputs = inputs * (attention_mask.unsqueeze(1).type_as(inputs))
+
+        if self.bidirectional and use_cache:
+            raise RuntimeError("Bidirectional EMA does not support incremental state")
+
+        if use_cache:
+            out, updated_state = self.ema_step(inputs, seq_len, past_state=prev_state)
+
+            # (batch_size X hidden_size) -> (1 x batch_size x hidden_size)
+            out = F.silu(out + residual)
+
+            # if incremental decoding, return the new state along with the output
+            return out, updated_state
+        else:
+            # (hidden_size x sequence_length)
+            kernel = self.get_ema_kernel(seq_len)
+            fft_len = seq_len
+            s_index = 0
+            kernel_size = kernel.size(1)
+            if self.bidirectional:
+                # split the kernel for each direction of EMA
+                k1, k2 = torch.split(kernel, [self.embed_dim, self.embed_dim], dim=0)
+                # (hidden_size X 2*sequence_length - 1)
+                kernel = F.pad(k1, (kernel_size - 1, 0)) + F.pad(k2.flip(-1), (0, kernel_size - 1))
+                inputs = F.pad(inputs, (kernel_size - 1, 0))
+                fft_len = fft_len + kernel_size - 1
+                s_index = 2 * kernel_size - 2
+
+            ema_output = self.fft_convolution(inputs, kernel, length=fft_len)[..., s_index : s_index + seq_len]
+            ema_output = ema_output.type_as(inputs)
+            # (batch_size X hidden_size X sequence_length) -> (sequence_length X batch_size X hidden_size)
+            gated_ema_output = F.silu(ema_output.permute(2, 0, 1) + residual)
+
+            return gated_ema_output, None
+
+
+class MegaGatedCrossAttention(nn.Module):
+    """
+    Gated Structured State Attention for use in encoder-decoder model. See Mega paper for more details. Only
+    modifications from original implementation are variable names, removing the unnecessary `before_attn_fn` and
+    `static_kv` arguments, and the stateful representation of incremental decoder state.
+    """
+
+    def __init__(self, config: MegaConfig):
+        super().__init__()
+
+        self.config = config
+        self.activation = ACT2FN[self.config.activation]
+        self.attention_activation = self.config.attention_activation
+        self.scaling = self.config.shared_representation_size**-0.5 if self.attention_activation == "softmax" else None
+
+        self.dropout = MegaDropout(self.config.dropout_prob, is_featurewise=self.config.use_feature_dropout)
+        self.hidden_dropout = MegaDropout(
+            self.config.hidden_dropout_prob, is_featurewise=self.config.use_feature_dropout
+        )
+        # Attention dropout is standard dropout
+        self.attention_dropout = MegaDropout(self.config.attention_probs_dropout_prob, is_featurewise=False)
+
+        self.prenorm = self.config.normalize_before_mega
+        self.norm = MegaSequenceNorm(
+            self.config.normalization_type, self.config.hidden_size, affine=self.config.norm_affine
+        )
+
+        self.k_proj = nn.Linear(self.config.hidden_size, self.config.shared_representation_size)
+        self.v_proj = nn.Linear(self.config.hidden_size, self.config.hidden_size)
+        self.q_proj = nn.Linear(
+            self.config.hidden_size, 2 * self.config.hidden_size + self.config.shared_representation_size
+        )
+        self.h_proj = nn.Linear(self.config.hidden_size, self.config.hidden_size)
+
+        if self.config.relative_positional_bias == "simple":
+            self.rel_pos_bias = MegaSimpleRelativePositionalBias(config)
+        elif self.config.relative_positional_bias == "rotary":
+            self.rel_pos_bias = MegaRotaryRelativePositionalBias(config)
+        else:
+            raise ValueError("unknown relative position bias: {}".format(self.config.relative_positional_bias))
+
+        self.softmax = nn.Softmax(dim=-1)
+
+    def element_attention(self, query, key, key_padding_mask, pidx):
+        bsz, src_len, _ = key.size()
+        tgt_len = query.size(1) if pidx is None else pidx + 1
+        if key_padding_mask is not None:
+            # (batch_size X source_sequence_length) --> (batch_size X 1 X 1)
+            lengths = key_padding_mask.sum(dim=-1).view(bsz, 1, 1)
+        else:
+            lengths = src_len
+
+        # (target_sequence_length X source_sequence_length)
+        bias = self.rel_pos_bias(max(tgt_len, src_len))[:, :src_len]
+        if pidx is not None:
+            if query.size(1) != 1:
+                raise ValueError("Position offset provided with queries longer than 1 token")
+            # source_sequence_length
+            bias = bias[pidx]
+        else:
+            # (target_sequence_length X source_sequence_length)
+            bias = bias[:tgt_len]
+
+        # (batch_size X target_sequence_length X source_sequence_length)
+        qk = torch.bmm(query, key.transpose(1, 2)) / lengths + bias
+
+        attn_weights = ACT2FN[self.attention_activation](qk).type_as(qk)
+
+        if key_padding_mask is not None:
+            attn_weights = attn_weights * key_padding_mask.unsqueeze(1)
+
+        return attn_weights
+
+    def softmax_attention(self, query, key, key_padding_mask, pidx):
+        bsz, src_len, _ = key.size()
+        tgt_len = query.size(1) if pidx is None else pidx + 1
+
+        # (target_sequence_length X source_sequence_length)
+        bias = self.rel_pos_bias(max(tgt_len, src_len))[:, :src_len]
+        if pidx is not None:
+            if query.size(1) != 1:
+                raise ValueError("Position offset provided with queries longer than 1 token")
+            # source_sequence_length
+            bias = bias[pidx]
+        else:
+            # (target_sequence_length X source_sequence_length)
+            bias = bias[:tgt_len]
+
+        # scaled attention
+        query = query * self.scaling
+        # (batch_size X target_sequence_length X source_sequence_length)
+        qk = torch.bmm(query, key.transpose(1, 2)) + bias
+
+        if key_padding_mask is not None:
+            qk = qk.masked_fill((1 - key_padding_mask).unsqueeze(1).to(torch.bool), float("-inf"))
+
+        attn_weights = self.softmax(qk).type_as(qk)
+        return attn_weights
+
+    def forward(
+        self,
+        query,
+        key: Optional[torch.Tensor],
+        value: Optional[torch.Tensor],
+        key_padding_mask: Optional[torch.Tensor] = None,
+        past_key_values: Optional[Tuple[torch.Tensor]] = None,
+        output_attentions: bool = False,
+        use_cache: bool = False,
+    ) -> Tuple[torch.Tensor, Optional[torch.Tensor]]:
+        """
+        Gated cross-attention used in Mega
+
+        Args:
+            query (`torch.Tensor` of shape `(target_sequence_length, batch_size, hidden_size)`):
+                The self (or target) sequence input used as query inputs for cross-attention
+            key (`torch.Tensor` of shape `(source_sequence_length, batch_size, hidden_size)`):
+                The cross (or source) sequence input with shape used as keys in cross-attention
+            value (`torch.Tensor` of shape `(source_sequence_length, batch_size, hidden_size)`):
+                The cross (or source) sequence input with shape used as values in cross-attention
+            key_padding_mask (`torch.LongTensor` of shape `(batch_size, source_sequence_length)`, *optional*):
+                Padding mask corresponding to the source sequence, where entries are 1 for *not masked* and 0 for
+                *masked* tokens
+            past_key_values (`tuple(torch.FloatTensor)`, *optional*):
+                If provided, the hidden state returned from the previous timestep during incremental decoding; expects
+                that prior cross-attention keys and values will be the last two items in the tuple
+            output_attentions (`bool`, defaults to `False`):
+                Whether or not to return the cross-attention weights.
+            use_cache (`bool`, defaults to `False`):
+                Whether to perfom incremental decoding; uses `prev_state` as the prior timestep, and returns the
+                updated EMA hidden state for use in the next step
+
+        Returns:
+            `tuple(torch.FloatTensor)` containing various elements depending on configuration ([`MegaConfig`]) and
+            inputs:
+            - **hidden_states** (`torch.FloatTensor` of shape `(target_sequence_length, batch_size, hidden_size)`) --
+              Hidden states from target sequence updated by gated cross-attention
+            - **attn_weights** (*optional*, returned when `output_attentions=True`) `torch.FloatTensor` of shape
+              `(batch_size, source_sequence_length, target_sequence_length)` -- The pairwise cross-attention weights
+              corresponding to each token in the source and target sequences
+            - **cross_key** (*optional*, returned when `use_cache=True`) `torch.FloatTensor` of shape `(batch_size,
+              source_sequence_length, config.shared_representation_size)` -- The cross-attention key state for use in
+              the next step of incremental decoding
+            - **cross_value** (*optional*, returned when `use_cache=True`) `torch.FloatTensor` of shape `(batch_size,
+              source_sequence_length, config.hidden_size)` -- The cross-attention value state for use in the next step
+              of incremental decoding
+        """
+
+        seq_len, bsz, embed_dim = query.size()
+        if embed_dim != self.config.hidden_size:
+            raise ValueError(
+                f"Unexpected embedding dimension received: input is {embed_dim} but expected {self.config.hidden_size}"
+            )
+
+        if past_key_values is not None:
+            # make sure the inputs only have a sequence length of 1 if we're doing incremental decoding
+            if seq_len != 1:
+                raise ValueError(f"Incremental decoding requested with self-sequence length > 1: {seq_len}")
+            # expect past_key_values to have (self_key, self_value, self_ema, cross_key, cross_value)
+            prev_cross_key, prev_cross_value = past_key_values[-2:]
+            key = value = None
+
+            # use the self-attention cache to get the position id of the current step
+            prev_self_key = past_key_values[0]
+            num_incremental_steps = prev_self_key.size(1) + 1
+        else:
+            prev_cross_key = prev_cross_value = None
+            # we still need the position id if we're doing incremental decoding (past_key_values will be None for the first step)
+            num_incremental_steps = 0 if use_cache and (seq_len == 1) else None
+
+        full_query = query
+        if self.prenorm:
+            full_query = self.norm(full_query)
+
+        # (target_sequence_length X batch_size X 2*hidden_size + shared_representation_size)
+        query_projected = self.q_proj(full_query)
+        # split the query projections into separate components
+        # - residual_weight is passed through sigmoid and sent through elementwise multiplication to the gated/weighted targets prior to being added to the query directly
+        # - target_gate is a silu-gated tensor that is multiplied by the attention-weighted target below prior to residual connection
+        # - attention_query is the part that is passed to the attention function
+        residual_weight, target_gate, attention_query = torch.split(
+            query_projected,
+            [self.config.hidden_size, self.config.hidden_size, self.config.shared_representation_size],
+            dim=-1,
+        )
+
+        # (target_sequence_length X batch_size X hidden_size)
+        residual_weight = torch.sigmoid(residual_weight)
+        target_gate = F.silu(target_gate)
+
+        if key is None:
+            if value is not None:
+                raise ValueError("Key and value must be `None` simultaneously")
+            projected_key = projected_value = None
+        else:
+            # (source_sequence_length X batch_size X shared_representation_size)
+            projected_key = self.k_proj(key)
+            # (source_sequence_length X batch_size X hidden_size)
+            projected_value = self.activation(self.v_proj(key))
+
+        # (target_sequence_length X batch_size X shared_representation_size)
+        # -> (batch_size X target_sequence_length X shared_representation_size)
+        attention_query = attention_query.transpose(0, 1)
+        if projected_key is not None:
+            projected_key = projected_key.transpose(0, 1)
+        if projected_value is not None:
+            projected_value = projected_value.transpose(0, 1)
+
+        # if we're doing incremental decoding, k and v are None and need to be overwritten with past values
+        if past_key_values is not None:
+            projected_key = prev_cross_key
+            projected_value = prev_cross_value
+
+        # if we're returning the cache for later use, store these now for later return (can be done without having past_key_values provided)
+        if use_cache:
+            updated_cross_key = projected_key
+            updated_cross_value = projected_value
+
+        ctx_len = projected_key.size(1)
+        # This is part of a workaround to get around fork/join parallelism
+        # not supporting Optional types.
+        if key_padding_mask is not None and key_padding_mask.dim() == 0:
+            key_padding_mask = None
+
+        if key_padding_mask is not None:
+            if key_padding_mask.size(0) != bsz:
+                raise ValueError("Key padding mask does not align on the batch dimension")
+            if key_padding_mask.size(1) != ctx_len:
+                raise ValueError("Key padding mask does not align on the sequence length dimension")
+
+        if self.attention_activation == "softmax":
+            attn_weights = self.softmax_attention(
+                attention_query, projected_key, key_padding_mask, num_incremental_steps
+            )
+        else:
+            attn_weights = self.element_attention(
+                attention_query, projected_key, key_padding_mask, num_incremental_steps
+            )
+
+        projected_value = self.hidden_dropout(projected_value, batch_first=True)
+        kernel = self.attention_dropout(attn_weights)
+        # (batch_size X target_sequence_length X hidden_size)
+        # -> (target_sequence_length X batch_size X hidden_size)
+        weighted_targets = torch.bmm(kernel, projected_value).transpose(0, 1)
+        # (target_sequence_length X batch_size X hidden_size)
+        weighted_targets = self.activation(self.h_proj(weighted_targets * target_gate))
+        weighted_targets = self.dropout(weighted_targets)
+        out = torch.addcmul(query, residual_weight, weighted_targets - query)
+
+        if not self.prenorm:
+            out = self.norm(out)
+
+        outputs = (out, attn_weights) if output_attentions else (out,)
+        if use_cache:
+            outputs = outputs + (updated_cross_key, updated_cross_value)
+
+        return outputs
+
+
+class MegaMovingAverageGatedAttention(nn.Module):
+    """
+    Pure PyTorch implementation of Mega block; see https://arxiv.org/abs/2209.10655 and original fairseq implementation
+    at https://github.com/facebookresearch/mega (copyright Meta Research, licensed under MIT License)
+
+    Differences from original implementation include hidden state refactor and fixed inconsistency with additive /
+    multiplicative attention masks
+    """
+
+    def __init__(self, config: MegaConfig):
+        super().__init__()
+        self.config = config
+        self.activation = ACT2FN[self.config.activation]
+        self.scaling = (
+            self.config.shared_representation_size**-0.5 if self.config.attention_activation == "softmax" else None
+        )
+        self.dropout = MegaDropout(self.config.dropout_prob, is_featurewise=self.config.use_feature_dropout)
+        self.hidden_dropout = MegaDropout(
+            self.config.hidden_dropout_prob, is_featurewise=self.config.use_feature_dropout
+        )
+        # attention dropout is standard dropout
+        self.attention_dropout = MegaDropout(self.config.attention_probs_dropout_prob, is_featurewise=False)
+
+        self.norm = MegaSequenceNorm(
+            self.config.normalization_type, self.config.hidden_size, affine=self.config.norm_affine
+        )
+        self.ema_gate = MegaMultiDimensionDampedEma(config)
+
+        self.v_proj = nn.Linear(self.config.hidden_size, self.config.intermediate_size)
+        self.mx_proj = nn.Linear(
+            self.config.hidden_size,
+            self.config.shared_representation_size + self.config.intermediate_size + 2 * self.config.hidden_size,
+        )
+        self.h_proj = nn.Linear(self.config.intermediate_size, self.config.hidden_size)
+
+        self.qk_weight = nn.Parameter(torch.Tensor(2, self.config.shared_representation_size))
+        self.qk_bias = nn.Parameter(torch.Tensor(2, self.config.shared_representation_size))
+
+        if self.config.relative_positional_bias == "simple":
+            self.rel_pos_bias = MegaSimpleRelativePositionalBias(config)
+        elif self.config.relative_positional_bias == "rotary":
+            self.rel_pos_bias = MegaRotaryRelativePositionalBias(config)
+        else:
+            raise ValueError(f"Unknown relative positional bias: {self.config.relative_positional_bias}")
+
+        self.softmax = nn.Softmax(dim=-1)
+        self.attention_function = (
+            self.softmax_attention if self.config.attention_activation == "softmax" else self.element_attention
+        )
+
+    def element_attention(self, query, key, padding_mask, causal_mask):
+        """
+        Apply element-wise attention via relu^2 or laplace. Same as original implementation but with standardized
+        causal attention mask. Expects the Hugging Face standard attention mask paradigm: 1 for not masked, and 0 for
+        masked.
+        """
+        seq_len = key.size(2)
+        if padding_mask is not None:
+            # (batch_size X number of chunks X 1)
+            lengths = padding_mask.sum(-1, keepdim=True)
+            # (batch_size X number of chunks X 1 X 1)
+            lengths = lengths.clamp(min=1.0).unsqueeze(-1)
+        else:
+            lengths = seq_len
+
+        if causal_mask is not None:
+            lengths = causal_mask.sum(dim=-1, keepdim=True)
+
+        # (sequence_length X sequence_length)
+        bias = self.rel_pos_bias(seq_len)
+        if seq_len != query.size(2):
+            if query.size(2) != 1:
+                raise ValueError("Size mismatch between Q and K in element attention")
+            # (1 X sequence_length)
+            bias = bias[-1:]
+
+        # (batch_size X number of chunks X sequence_length X sequence_length)
+        qk = torch.matmul(query, key.transpose(2, 3)) / lengths + bias
+
+        attn_weights = ACT2FN[self.config.attention_activation](qk).type_as(qk)
+
+        if padding_mask is not None:
+            attn_weights = attn_weights * padding_mask.unsqueeze(2)
+
+        if causal_mask is not None:
+            attn_weights = attn_weights * causal_mask
+
+        return attn_weights
+
+    def softmax_attention(self, query, key, padding_mask, causal_mask):
+        "Standard softmax self-attention, as in the original Transformer paper"
+        seq_len = key.size(2)
+        # (sequence_length X sequence_length)
+        bias = self.rel_pos_bias(seq_len)
+        if seq_len != query.size(2):
+            if query.size(2) != 1:
+                raise ValueError("Size mismatch between Q and K in softmax attention")
+            # (1 X sequence_length)
+            bias = bias[-1:]
+
+        # scaled attention
+        query = query * self.scaling
+
+        # (batch_size x number of chunks x chunk_size x chunk_size) if chunking
+        # (batch_size x 1 x sequence_length x sequence_length) otherwise
+        qk = torch.matmul(query, key.transpose(2, 3)) + bias
+
+        # apply causal mask (presumed to be 1/0 for not masked / masked)
+        # additive, but convert to 0/-inf (which is not explicitly in the Mega source code)
+        if causal_mask is not None:
+            additive_causal_mask = torch.zeros_like(causal_mask, dtype=qk.dtype)
+            additive_causal_mask = additive_causal_mask.masked_fill((1 - causal_mask).bool(), float("-inf"))
+            qk = qk + additive_causal_mask
+
+        if padding_mask is not None:
+            # 1 for tokens which are *not masked*
+            # 0 for tokens which are *masked*
+            # replace masked tokens with -inf to make softmax ignore them
+            # need to invert the padding mask to match what mega original did
+            padding_mask = 1 - padding_mask
+            padding_mask_all = padding_mask.all(dim=-1, keepdim=True)
+            padding_mask = torch.logical_and(padding_mask, ~padding_mask_all)
+            qk = qk.masked_fill(padding_mask.unsqueeze(2).to(torch.bool), float("-inf"))
+
+        attn_weights = self.softmax(qk).type_as(qk)
+        return attn_weights
+
+    def forward(
+        self,
+        input,
+        padding_mask: Optional[torch.Tensor] = None,
+        causal_mask: Optional[torch.Tensor] = None,
+        past_key_values: Optional[Tuple[torch.Tensor]] = None,
+        output_attentions=False,
+        use_cache=False,
+    ):
+        """
+        Mega's self-attention block, which combines multi-headed EMA with traditional self-attention
+
+        Args:
+            input (`torch.Tensor` of shape `(sequence_length, batch_size, hidden_size)`):
+                Hidden states to be updated by Mega's self-attention
+            padding_mask (`torch.LongTensor` of shape `(batch_size, sequence_length)`, *optional*):
+                Indicates which inputs are to be ignored due to padding, where elements are either 1 for *not masked*
+                or 0 for *masked*
+            causal_mask (`torch.LongTensor` of shape `(sequence_length, sequence_length)`, *optional*):
+                Indicates which inputs are to be ignored due to causal attention, where elements are either 1 for *not
+                masked* or 0 for *masked*
+            past_key_values (`tuple(torch.Tensor)`, *optional*):
+                The hidden states returned from the previous timestep during incremental decoding; expects that
+                self-attention key, value, and EMA states are the first 3 entries in the tuple
+            output_attentions (`bool`, default `False`):
+                Whether to return self-attention weights
+            use_cache (`bool`, default `False`):
+                Whether to perfom incremental decoding; uses `past_key_values` as prior state, and returns the updated
+                states for use in the next step
+
+        Returns:
+            `tuple(torch.FloatTensor)` containing various elements depending on configuration ([`MegaConfig`]) and
+            inputs:
+            - **hidden_states** (`torch.FloatTensor` of shape `(sequence_length, batch_size, hidden_size)`) -- Hidden
+              states from target sequence updated by Mega's self-attention
+            - **attn_weights** (*optional*, returned when `output_attentions=True`) `torch.FloatTensor` of shape
+              `(batch_size, 1, sequence_length, sequence_length)` -- The self-attention weights corresponding to how
+              each token in the input sequence attends to every other token
+            - **self_key** (*optional*, returned when `use_cache=True`) `torch.FloatTensor` of shape `(batch_size,
+              sequence_length, config.shared_representation_size)` -- The self-attention key state for use in the next
+              step of incremental decoding
+            - **self_value** (*optional*, returned when `use_cache=True`) `torch.FloatTensor` of shape `(batch_size,
+              sequence_length, config.hidden_size)` -- The self-attention value state for use in the next step of
+              incremental decoding
+            - **self_ema_state** (*optional*, returned when `use_cache=True`) `torch.FloatTensor` of shape
+              `(batch_size, config.ndim)` The incremental EMA state for use in the next step of incremental decoding.
+        """
+
+        seq_len, bsz, embed_dim = input.size()
+        if embed_dim != self.config.hidden_size:
+            raise ValueError(f"Input embedding dimension should be {self.config.hidden_size}; received {embed_dim}")
+
+        # store inputs for residual connection and handle pre-norm if requested
+        residual = input
+        if self.config.normalize_before_mega:
+            input = self.norm(input)
+
+        # (sequence_length X batch_size X hidden_size) -> (sequence_length X batch_size X intermediate_size)
+        value = self.activation(self.v_proj(input))
+
+        # unpack the incremental state if provided
+        # assumed to be (self K, self V, self EMA state, cross K, cross V)
+        # also assumes that incremental decoding is working one token at a time, so input sequence length must be 1
+        if self.config.is_decoder and (past_key_values is not None):
+            if seq_len > 1:
+                raise ValueError(f"Incremental decoding only supports self sequence length of 1; received {seq_len}")
+            # the first 3 items in the saved states will be these regardless of whether cross-attention is present
+            prev_self_key, prev_self_value, prev_ema_state = past_key_values[0:3]
+        else:
+            prev_self_key = prev_self_value = prev_ema_state = None
+
+        # ema output is (sequence_length x batch_size x hidden_size)
+        # updated_ema_state will be None if use_cache=False; otherwise (batch_size, config.ndim)
+        ema_out, updated_ema_state = self.ema_gate(
+            input, attention_mask=padding_mask, prev_state=prev_ema_state, use_cache=use_cache
+        )
+        ema_out = self.dropout(ema_out)
+
+        # (sequence_length X batch_size X hidden_size)
+        # -> (sequence_length X batch_size X 2*hidden_size + config.shared_representation_size + config.intermediate_size)
+        # - residual_weight -> sigmoid -> applied to residual connection in torch.addcmul
+        # - query_key_gates -> split into two components: query_key becomes query and key for attention input, gates becomes gating for self-attention output
+        # - intermediate_state -> added to weighted attention output, sent through activation, and has inputs subtracted during
+        #   torch.addcmul to create the final layer output
+        base = self.mx_proj(ema_out)
+        residual_weight, query_key_gates, intermediate_state = torch.split(
+            base,
+            [
+                self.config.hidden_size,
+                self.config.shared_representation_size + self.config.intermediate_size,
+                self.config.hidden_size,
+            ],
+            dim=-1,
+        )
+
+        # (sequence_length X batch_size X hidden_size)
+        residual_weight = torch.sigmoid(residual_weight)
+
+        # (sequence_length X batch_size X shared_representation_size + intermediate_size)
+        query_key_gates = F.silu(query_key_gates)
+
+        # split into two different tensors: one for Q/K usage and the other for gating self-attention
+        query_key, attention_gate = torch.split(
+            query_key_gates, [self.config.shared_representation_size, self.config.intermediate_size], dim=-1
+        )
+
+        # (sequence_length X batch_size X shared_representation_size)
+        # -> (sequence_length X batch_size X 1 X shared_representation_size)
+        # -> (sequence_length X batch_size X 2 X shared_representation_size)
+        query_key = query_key.unsqueeze(2) * self.qk_weight + self.qk_bias
+
+        # (sequence_length X batch_size X 2 X shared_representation_size)
+        # -> 2 tensors of (sequence_length X batch_size X shared_representation_size)
+        query, key = torch.unbind(query_key, dim=2)
+
+        # (sequence_length X batch_size X dimension)
+        # -> (batch_size X sequence_length X dimension)
+        # where `dimension` is either shared_representation_size (queries and keys) or intermediate_size (values)
+        query = query.transpose(0, 1)
+        key = key.transpose(0, 1)
+        value = value.transpose(0, 1)
+
+        if self.config.is_decoder:
+            # combine history and current to save updated state (if history is provided)
+            # when chunking is applied, the past states will be None at the end of the chunk, in
+            # which case, proceed as if no K/V history had been provided
+            # saved states are stored with shape (batch_size X sequence_length X dimension)
+            if prev_self_key is not None:
+                key = torch.cat([prev_self_key, key], dim=1)
+            if prev_self_value is not None:
+                value = torch.cat([prev_self_value, value], dim=1)
+
+            # if not chunking, store as-is
+            if not self.config.use_chunking:
+                updated_self_key = key
+                updated_self_value = value
+            else:
+                curr_len = key.size(1) % self.config.chunk_size
+                if curr_len == 0:
+                    # if we're chunking and have reached the end of a chunk, wipe out the saved state
+                    updated_self_key = None
+                    updated_self_value = None
+                else:
+                    updated_self_key = key
+                    updated_self_value = value
+
+        ctx_len = key.size(1)  # potentially differs from seq_len because of incremental decoding
+        if not self.config.use_chunking:
+            # if we're not chunking, treat the entire sequence as one long chunk
+            # (batch_size X sequence_length X dimension) -> (batch_size X 1 X sequence_length X dimension)
+            query = query.unsqueeze(1)
+            key = key.unsqueeze(1)
+            value = value.unsqueeze(1)
+            if padding_mask is not None:
+                # (batch_size X sequence_length) -> (batch_size X 1 X sequence_length)
+                padding_mask = padding_mask.unsqueeze(1)
+        else:
+            # otherwise, split the sequences in the batch into `n_chunks` chunks of size `chunk_size`
+            if seq_len < self.config.chunk_size:
+                query = query.unsqueeze(1)
+            else:
+                # (batch_size X sequence_length X dimension) -> (batch_size X n_chunks X chunk_size X dimension)
+                n_chunks = seq_len // self.config.chunk_size
+                query = query.reshape(bsz, n_chunks, self.config.chunk_size, self.config.shared_representation_size)
+
+            if ctx_len < self.config.chunk_size:
+                key = key.unsqueeze(1)
+                value = value.unsqueeze(1)
+                if padding_mask is not None:
+                    padding_mask = padding_mask.unsqueeze(1)
+            else:
+                # (batch_size X sequence_length X dimension) -> (batch_size X n_chunks X chunk_size X dimension)
+                n_chunks = ctx_len // self.config.chunk_size
+                key = key.reshape(bsz, n_chunks, self.config.chunk_size, self.config.shared_representation_size)
+                value = value.reshape(bsz, n_chunks, self.config.chunk_size, self.config.intermediate_size)
+                if padding_mask is not None:
+                    padding_mask = padding_mask.view(bsz, n_chunks, self.config.chunk_size)
+
+        # this is in the original Mega implementation to work around fork/join parallelism not supporting optional types
+        if padding_mask is not None and padding_mask.dim() == 0:
+            padding_mask = None
+
+        attn_weights = self.attention_function(query, key, padding_mask=padding_mask, causal_mask=causal_mask)
+
+        value = self.hidden_dropout(value, batch_first=True)
+        kernel = self.attention_dropout(attn_weights)
+
+        # (batch_size x n_chunks x chunk_size x intermediate_size) -> (sequence_length X batch_size X intermediate_size)
+        weighted_self_output = (
+            torch.matmul(kernel, value).view(bsz, seq_len, self.config.intermediate_size).transpose(0, 1)
+        )
+
+        # (sequence_length X batch_size X intermediate_size) -> (sequence_length X batch_size X hidden_size)
+        weighted_self_output = self.activation(intermediate_state + self.h_proj(weighted_self_output * attention_gate))
+        weighted_self_output = self.dropout(weighted_self_output)
+        # (sequence_length X batch_size X hidden_size)
+        out = torch.addcmul(residual, residual_weight, weighted_self_output - residual)
+
+        if not self.config.normalize_before_mega:
+            out = self.norm(out)
+
+        return_values = (out, attn_weights) if output_attentions else (out,)
+
+        if self.config.is_decoder:
+            return_values = return_values + (updated_self_key, updated_self_value, updated_ema_state)
+
+        return return_values
+
+
+class MegaNormalizedFeedForwardNetwork(nn.Module):
+    """
+    Normalized feed-forward network used in Mega blocks. Left as-is from original Mega repo aside from retrieving args
+    from Hugging Face config
+    """
+
+    def __init__(self, config: MegaConfig):
+        super().__init__()
+
+        self.config = config
+        self.hidden_dim = config.nffn_hidden_size
+        self.act_fn = config.activation
+        self.activation = ACT2FN[config.activation]
+
+        self.dropout = MegaDropout(self.config.dropout_prob, is_featurewise=self.config.use_feature_dropout)
+        self.hidden_dropout = MegaDropout(
+            self.config.nffn_activation_dropout_prob, is_featurewise=self.config.use_feature_dropout
+        )
+
+        self.prenorm = self.config.normalize_before_ffn
+        self.norm = MegaSequenceNorm(
+            self.config.normalization_type, self.config.hidden_size, affine=self.config.norm_affine
+        )
+
+        self.fc1 = nn.Linear(self.config.hidden_size, self.config.nffn_hidden_size)
+        self.fc2 = nn.Linear(self.config.nffn_hidden_size, self.config.hidden_size)
+
+    def forward(self, inputs):
+        residual = inputs
+
+        if self.prenorm:
+            inputs = self.norm(inputs)
+
+        hidden = self.activation(self.fc1(inputs))
+        hidden = self.hidden_dropout(hidden)
+        output = self.fc2(hidden)
+        output = self.dropout(output)
+        output = output + residual
+
+        if not self.prenorm:
+            output = self.norm(output)
+
+        return output
+
+
+class MegaBlock(nn.Module):
+    def __init__(self, config: MegaConfig):
+        super().__init__()
+        self.seq_len_dim = 1
+        self.mega_layer = MegaMovingAverageGatedAttention(config)
+        self.nffn = MegaNormalizedFeedForwardNetwork(config) if config.use_normalized_ffn else None
+        self.is_decoder = config.is_decoder
+        self.add_cross_attention = config.add_cross_attention
+        if self.add_cross_attention:
+            if not self.is_decoder:
+                raise ValueError(f"{self} should be used as a decoder model if cross attention is added")
+            self.cross_attn = MegaGatedCrossAttention(config)
+        else:
+            self.cross_attn = None
+
+    def forward(
+        self,
+        hidden_states: torch.Tensor,
+        attention_mask: Optional[torch.LongTensor] = None,
+        causal_mask: Optional[torch.LongTensor] = None,
+        encoder_hidden_states: Optional[torch.FloatTensor] = None,
+        encoder_attention_mask: Optional[torch.FloatTensor] = None,
+        past_key_value: Optional[Tuple[torch.FloatTensor]] = None,
+        output_attentions: Optional[bool] = False,
+        use_cache: bool = False,
+    ) -> Tuple[torch.Tensor]:
+        """
+        A single Mega layer: either encoder or decoder, with optional cross-attention and optional normalized
+        feed-forward layer
+
+        Args:
+            hidden_states (`torch.Tensor` of shape `(target_sequence_length, batch_size, hidden_size)`):
+                Hidden states to be updated by the Mega block
+            attention_mask (`torch.LongTensor` of shape `(batch_size, target_sequence_length)`, *optional*):
+                Indicates which entries in the self/target sequence are to be ignored (mostly due to padding), where
+                elements are either 1 for *not masked* or 0 for *masked*. Causal attention is enforced internally.
+            causal_mask (`torch.LongTensor` of shape `(sequence_length, sequence_length)`, *optional*):
+                Indicates which inputs are to be ignored due to causal attention, where elements are either 1 for *not
+                masked* or 0 for *masked*
+            encoder_hidden_states (`torch.Tensor`, of shape `(source_sequence_length, batch_size, hidden_size)`, *optional*):
+                Encoder hidden states to be used for cross-attention (and required for encoder-decoder model setup)
+            encoder_attention_mask (`torch.LongTensor` of shape `(batch_size, source_sequence_length)`, *optional*):
+                Indicates which entries in the cross/source sequence are to be ignored (mostly due to padding), where
+                elements are either 1 for *not masked* or 0 for *masked*.
+            past_key_value (`tuple(torch.Tensor)`, *optional*):
+                The hidden states returned from the previous timestep during incremental decoding; expects that
+                self-attention key, value, and EMA states are the first 3 entries in the tuple, and (if doing
+                cross-attention) cross-attention key and value are the last 2 entries in the tuple
+            output_attentions (`bool`, default `False`):
+                Whether to return self-attention weights
+            use_cache (`bool`, default `False`):
+                Whether to perfom incremental decoding; uses `past_key_value` as prior state, and returns the updated
+                states for use in the next step
+
+        Returns:
+            `tuple(torch.FloatTensor)` containing various elements depending on configuration ([`MegaConfig`]) and
+            inputs:
+            - **hidden_states** (`torch.FloatTensor` of shape `(target_sequence_length, batch_size, hidden_size)`) --
+              Hidden states from target sequence updated by Mega
+            - **self_attn_weights** (*optional*, returned when `output_attentions=True`) `torch.FloatTensor` of shape
+              `(batch_size, 1, target_sequence_length, target_sequence_length)` -- The self-attention weights
+              corresponding to how each token in the input sequence attends to every other token
+            - **cross_attn_weights** (*optional*, returned when `output_attentions=True` and
+              `config.add_cross_attention=True`) `torch.FloatTensor` of shape `(batch_size, source_sequence_length,
+              target_sequence_length)` -- Pairwise cross-attention weights between every entry in the source sequence
+              and target sequence
+            - **self_key** (*optional*, returned when `use_cache=True`) `torch.FloatTensor` of shape `(batch_size,
+              sequence_length, config.shared_representation_size)` -- The self-attention key state for use in the next
+              step of incremental decoding
+            - **self_value** (*optional*, returned when `use_cache=True`) `torch.FloatTensor` of shape `(batch_size,
+              sequence_length, config.hidden_size)` -- The self-attention value state for use in the next step of
+              incremental decoding
+            - **self_ema_state** (*optional*, returned when `use_cache=True`) `torch.FloatTensor` of shape
+              `(batch_size, config.ndim)` The incremental EMA state for use in the next step of incremental decoding.
+            - **cross_key** (*optional*, returned when `use_cache=True` and `config.is_decoder=True`)
+              `torch.FloatTensor` of shape `(batch_size, source_sequence_length, config.shared_representation_size)` --
+              The cross-attention key state for use in the next step of incremental decoding
+            - **cross_value** (*optional*, returned when `use_cache=True` and `config.is_decoder=True`)
+              `torch.FloatTensor` of shape `(batch_size, source_sequence_length, config.hidden_size)` -- The
+              cross-attention value state for use in the next step of incremental decoding
+        """
+
+        # incremental decoding in the MegaMultiDimensionDampedEma module requires that the attention mask has the same
+        # sequence length as the input tensor; if we're caching incremental states, we assume the input
+        # sequence length is 1 (Mega will break otherwise), so we take the padding mask for the final
+        # token in the input (mask is received as [batch X sequence length])
+        if use_cache and (past_key_value is not None) and (attention_mask is not None):
+            mega_padding_mask = attention_mask[:, -1].unsqueeze(-1)
+        else:
+            mega_padding_mask = attention_mask
+
+        mega_outputs = self.mega_layer(
+            input=hidden_states,
+            padding_mask=mega_padding_mask,
+            causal_mask=causal_mask,
+            past_key_values=past_key_value,
+            output_attentions=output_attentions,
+            use_cache=use_cache,
+        )
+
+        new_hidden_states = mega_outputs[0]
+        self_key, self_value, self_ema_state = mega_outputs[-3:] if use_cache else (None, None, None)
+        self_attention_weights = mega_outputs[1] if output_attentions else None
+
+        # optional cross attention
+        if self.cross_attn is not None:
+            if encoder_hidden_states is None:
+                raise ValueError("Requested cross-attention without providing encoder hidden states")
+
+            cross_attn_outputs = self.cross_attn(
+                query=new_hidden_states,
+                key=encoder_hidden_states,
+                value=encoder_hidden_states,
+                key_padding_mask=encoder_attention_mask,
+                past_key_values=past_key_value,
+                output_attentions=output_attentions,
+                use_cache=use_cache,
+            )
+
+            # update the hidden state from cross attention
+            new_hidden_states = cross_attn_outputs[0]
+            # store cross-attention k/v if caching
+            cross_key, cross_value = cross_attn_outputs[-2:] if use_cache else (None, None)
+            cross_attention_weights = cross_attn_outputs[1] if output_attentions else None
+
+        # optional NFFN follows cross attention
+        if self.nffn is not None:
+            new_hidden_states = self.nffn(new_hidden_states)
+
+        outs = (new_hidden_states,)
+        if output_attentions:
+            outs = outs + (self_attention_weights,)
+            if self.cross_attn is not None:
+                outs = outs + (cross_attention_weights,)
+
+        if use_cache:
+            new_key_values = (
+                self_key,
+                self_value,
+                self_ema_state,
+            )
+            if self.cross_attn is not None:
+                new_key_values = new_key_values + (cross_key, cross_value)
+
+            outs = outs + (new_key_values,)
+
+        return outs
+
+
+# copied from transformers.models.roberta.modeling_roberta.RobertaPooler with Roberta->Mega
+class MegaPooler(nn.Module):
+    def __init__(self, config):
+        super().__init__()
+        self.dense = nn.Linear(config.hidden_size, config.hidden_size)
+        self.activation = nn.Tanh()
+
+    def forward(self, hidden_states: torch.Tensor) -> torch.Tensor:
+        # We "pool" the model by simply taking the hidden state corresponding
+        # to the first token.
+        first_token_tensor = hidden_states[:, 0]
+        pooled_output = self.dense(first_token_tensor)
+        pooled_output = self.activation(pooled_output)
+        return pooled_output
+
+
+class MegaPreTrainedModel(PreTrainedModel):
+    """
+    An abstract class to handle weights initialization and a simple interface for downloading and loading pretrained
+    models.
+    """
+
+    config_class = MegaConfig
+    base_model_prefix = "mega"
+    supports_gradient_checkpointing = False
+    _no_split_modules = ["MegaMovingAverageGatedAttention"]
+
+    def _init_weights(self, module):
+        """Initialize the weights"""
+        if isinstance(module, MegaMultiDimensionDampedEma):
+            with torch.no_grad():
+                # delta & alpha
+                nn.init.normal_(module.damping_factor, mean=0.0, std=self.config.ema_delta_alpha_range)
+                nn.init.normal_(module.decay_factor, mean=0.0, std=self.config.ema_delta_alpha_range)
+                # beta [1, -1, 1, -1, ...] seems more stable.
+                val = torch.ones(self.config.ema_projection_size, 1)
+                if self.config.ema_projection_size > 1:
+                    idx = torch.tensor(list(range(1, self.config.ema_projection_size, 2)))
+                    val.index_fill_(0, idx, -1.0)
+                module.ema_expansion_matrix.normal_(mean=0.0, std=self.config.ema_beta_range).add_(val)
+                # gamma & omega
+                nn.init.normal_(module.kernel_projection_matrix, mean=0.0, std=self.config.ema_gamma_omega_range)
+                nn.init.normal_(module.residual_weight, mean=0.0, std=self.config.ema_gamma_omega_range)
+        elif isinstance(module, MegaSimpleRelativePositionalBias):
+            nn.init.normal_(module.rel_pos_bias, mean=0.0, std=self.config.initializer_range)
+        elif isinstance(module, MegaRotaryRelativePositionalBias):
+            nn.init.normal_(module.alpha, mean=0.0, std=self.config.initializer_range)
+            nn.init.normal_(module.b_param, mean=0.0, std=self.config.initializer_range)
+        elif isinstance(module, MegaScaleNorm):
+            if self.config.norm_affine:
+                nn.init.constant_(module.scalar, 1.0)
+        elif isinstance(module, MegaRMSNorm):
+            if self.config.norm_affine:
+                nn.init.constant_(module.weight, 1.0)
+        elif isinstance(module, MegaMovingAverageGatedAttention):
+            # linear layers covered separately by the generic nn.Linear init below
+            nn.init.normal_(module.qk_weight, mean=0.0, std=self.config.initializer_range)
+            nn.init.constant_(module.qk_bias, 0.0)
+        elif isinstance(module, nn.Linear):
+            # initializes all linear layers in the entire network
+            module.weight.data.normal_(mean=0.0, std=self.config.initializer_range)
+            if module.bias is not None:
+                module.bias.data.zero_()
+        elif isinstance(module, nn.Embedding):
+            module.weight.data.normal_(mean=0.0, std=self.config.initializer_range)
+            if module.padding_idx is not None:
+                module.weight.data[module.padding_idx].zero_()
+        elif isinstance(module, nn.LayerNorm):
+            module.bias.data.zero_()
+            module.weight.data.fill_(1.0)
+
+
+MEGA_START_DOCSTRING = r"""
+
+    This model inherits from [`PreTrainedModel`]. Check the superclass documentation for the generic methods the
+    library implements for all its model (such as downloading or saving, resizing the input embeddings, pruning heads
+    etc.)
+
+    This model is also a PyTorch [torch.nn.Module](https://pytorch.org/docs/stable/nn.html#torch.nn.Module) subclass.
+    Use it as a regular PyTorch Module and refer to the PyTorch documentation for all matter related to general usage
+    and behavior.
+
+    Parameters:
+        config ([`MegaConfig`]): Model configuration class with all the parameters of the
+            model. Initializing with a config file does not load the weights associated with the model, only the
+            configuration. Check out the [`~PreTrainedModel.from_pretrained`] method to load the model weights.
+"""
+
+MEGA_INPUTS_DOCSTRING = r"""
+    Args:
+        input_ids (`torch.LongTensor` of shape `({0})`):
+            Indices of input sequence tokens in the vocabulary.
+
+            Indices can be obtained using [`AutoTokenizer`]. See [`PreTrainedTokenizer.encode`] and
+            [`PreTrainedTokenizer.__call__`] for details.
+
+            [What are input IDs?](../glossary#input-ids)
+        attention_mask (`torch.FloatTensor` of shape `({0})`, *optional*):
+            Mask to avoid performing attention on padding token indices. Mask values selected in `[0, 1]`:
+
+            - 1 for tokens that are **not masked**,
+            - 0 for tokens that are **masked**.
+
+            [What are attention masks?](../glossary#attention-mask)
+        token_type_ids (`torch.LongTensor` of shape `({0})`, *optional*):
+            Segment token indices to indicate first and second portions of the inputs. Indices are selected in `[0,1]`:
+
+            - 0 corresponds to a *sentence A* token,
+            - 1 corresponds to a *sentence B* token.
+            This parameter can only be used when the model is initialized with `add_token_type_embeddings` parameter
+            set to `True`. All the value in this tensor should be always < config.type_vocab_size.
+
+            [What are token type IDs?](../glossary#token-type-ids)
+        inputs_embeds (`torch.FloatTensor` of shape `({0}, hidden_size)`, *optional*):
+            Optionally, instead of passing `input_ids` you can choose to directly pass an embedded representation. This
+            is useful if you want more control over how to convert `input_ids` indices into associated vectors than the
+            model's internal embedding lookup matrix.
+        output_attentions (`bool`, *optional*):
+            Whether or not to return the attentions tensors of all attention layers. See `attentions` under returned
+            tensors for more detail.
+        output_hidden_states (`bool`, *optional*):
+            Whether or not to return the hidden states of all layers. See `hidden_states` under returned tensors for
+            more detail.
+        return_dict (`bool`, *optional*):
+            Whether or not to return a [`~utils.ModelOutput`] instead of a plain tuple.
+"""
+
+
+@add_start_docstrings(
+    "The bare MEGA Model transformer outputting raw hidden-states without any specific head on top.",
+    MEGA_START_DOCSTRING,
+)
+class MegaModel(MegaPreTrainedModel):
+    """
+
+    The model can behave as an encoder (with only self-attention) as well as a decoder, in which case a layer of
+    cross-attention is added after self-attention, following the architecture described in *Mega: Moving Average
+    Equipped Gated Attention*_ by Xuezhe Ma, Chunting Zhou, Xiang Kong, Junxian He, Liangke Gui, Graham Neubig,
+    Jonathan May, and Luke Zettlemoyer
+
+    To behave as a decoder the model needs to be initialized with the `is_decoder` argument of the configuration set to
+    `True` and `bidirectional` set to `False`. To be used in a Seq2Seq model, the model needs to initialized with both
+    `is_decoder=True` and `bidirectional=False` argument as well as `add_cross_attention` set to `True`; an
+    `encoder_hidden_states` is then expected as an input to the forward pass.
+
+    .. _*Mega: Moving Average Equipped Gated Attention*: https://arxiv.org/abs/2209.10655
+
+    """
+
+    def __init__(self, config: MegaConfig, add_pooling_layer=True):
+        super().__init__(config)
+        self.config = config
+
+        self.embedding_layer = MegaEmbeddings(config)
+        self.layers = nn.ModuleList([MegaBlock(config) for _ in range(config.num_hidden_layers)])
+
+        self.pooler = MegaPooler(config) if add_pooling_layer else None
+
+        # Initialize weights and apply final processing (retained from RoBERTa code)
+        self.post_init()
+
+    def get_input_embeddings(self):
+        return self.embedding_layer.word_embeddings
+
+    def set_input_embeddings(self, value):
+        self.embedding_layer.word_embeddings = value
+
+    @add_start_docstrings_to_model_forward(MEGA_INPUTS_DOCSTRING.format("batch_size, sequence_length"))
+    @add_code_sample_docstrings(
+        checkpoint=_CHECKPOINT_FOR_DOC,
+        output_type=BaseModelOutputWithPoolingAndCrossAttentions,
+        config_class=_CONFIG_FOR_DOC,
+    )
+    def forward(
+        self,
+        input_ids: Optional[torch.Tensor] = None,
+        attention_mask: Optional[torch.Tensor] = None,
+        token_type_ids: Optional[torch.Tensor] = None,
+        inputs_embeds: Optional[torch.Tensor] = None,
+        encoder_hidden_states: Optional[torch.Tensor] = None,
+        encoder_attention_mask: Optional[torch.Tensor] = None,
+        past_key_values: Optional[List[torch.FloatTensor]] = None,
+        use_cache: Optional[bool] = None,
+        output_attentions: Optional[bool] = None,
+        output_hidden_states: Optional[bool] = None,
+        return_dict: Optional[bool] = None,
+    ) -> Union[Tuple[torch.Tensor], BaseModelOutputWithPoolingAndCrossAttentions]:
+        r"""
+        encoder_hidden_states  (`torch.FloatTensor` of shape `(batch_size, sequence_length, hidden_size)`, *optional*):
+            Sequence of hidden-states at the output of the last layer of the encoder. Used in the cross-attention if
+            the model is configured as a decoder.
+        encoder_attention_mask (`torch.FloatTensor` of shape `(batch_size, sequence_length)`, *optional*):
+            Mask to avoid performing attention on the padding token indices of the encoder input. This mask is used in
+            the cross-attention if the model is configured as a decoder. Mask values selected in `[0, 1]`:
+
+            - 1 for tokens that are **not masked**,
+            - 0 for tokens that are **masked**.
+        past_key_values (`tuple(tuple(torch.FloatTensor))` of length `config.n_layers` with each tuple having 4 tensors of shape `(batch_size, num_heads, sequence_length - 1, embed_size_per_head)`):
+            Contains precomputed key and value hidden states of the attention blocks. Can be used to speed up decoding.
+
+            If `past_key_values` are used, the user can optionally input only the last `decoder_input_ids` (those that
+            don't have their past key value states given to this model) of shape `(batch_size, 1)` instead of all
+            `decoder_input_ids` of shape `(batch_size, sequence_length)`.
+        use_cache (`bool`, *optional*):
+            If set to `True`, `past_key_values` key value states are returned and can be used to speed up decoding (see
+            `past_key_values`).
+        """
+
+        output_attentions = output_attentions if output_attentions is not None else self.config.output_attentions
+        output_hidden_states = (
+            output_hidden_states if output_hidden_states is not None else self.config.output_hidden_states
+        )
+        return_dict = return_dict if return_dict is not None else self.config.use_return_dict
+
+        if input_ids is not None and inputs_embeds is not None:
+            raise ValueError("You cannot specify both input_ids and inputs_embeds at the same time")
+        elif input_ids is not None:
+            self.warn_if_padding_and_no_attention_mask(input_ids, attention_mask)
+            input_shape = input_ids.size()
+            device = input_ids.device
+        elif inputs_embeds is not None:
+            input_shape = inputs_embeds.size()[:-1]
+            device = inputs_embeds.device
+        else:
+            raise ValueError("You have to specify either input_ids or inputs_embeds")
+
+        if self.config.use_chunking:
+            input_shape = torch.tensor([input_shape[0], self.config.chunk_size])
+
+        batch_size, sequence_length = input_shape
+
+        if self.config.use_chunking and (sequence_length > self.config.chunk_size):
+            if sequence_length % self.config.chunk_size != 0:
+                raise ValueError(
+                    f"config.use_chunking is activated; input sequence length must be shorter than or a multiple of config.chunk_size\nreceived sequence length of {sequence_length} with chunk size {self.config.chunk_size}"
+                )
+
+        if self.config.is_decoder:
+            use_cache = use_cache if use_cache is not None else self.config.use_cache
+
+            # Mega expects the causal mask to be a 2D square matrix of (from) x (to) over the input sequence length
+            # the HF utility function generates a 3D causal mask which includes batch size, so we'll create a dummy
+            # mask with the correct device and all ones
+            temp_mask_for_extension = torch.ones((1, sequence_length), dtype=torch.long, device=device)
+            causal_mask = self.create_extended_attention_mask_for_decoder(input_shape, temp_mask_for_extension)
+
+            # get rid of batch dimension in the generated mask; result is (sequence_length X sequence_length)
+            causal_mask = causal_mask.squeeze(0)
+        else:
+            use_cache = False
+            causal_mask = None
+
+        # if using cache, make sure we have a tuple of tuples which matches the length of our hidden layers
+        if (past_key_values is not None) and (len(past_key_values) != self.config.num_hidden_layers):
+            raise ValueError(
+                f"Received past key/value cache with size mismatch; expected {self.config.num_hidden_layers}, received {len(past_key_values)}"
+            )
+
+        # get embeddings (batch X sequence length X embed dim)
+        embedding_output = self.embedding_layer(
+            input_ids=input_ids, token_type_ids=token_type_ids, inputs_embeds=inputs_embeds
+        )
+
+        # transpose for Mega --> (seq len X batch X embed dim)
+        hidden_states = embedding_output.transpose(0, 1)
+
+        # we expect encoder hidden states to also have batch first in line
+        # with typical Hugging Face behavior (which is also how we return them)
+        # Mega expects sequence length first, so do the same transpose here
+        if encoder_hidden_states is not None:
+            encoder_hidden_states = encoder_hidden_states.transpose(0, 1)
+
+        # pass through mega layers
+        all_hidden_states = (embedding_output,) if output_hidden_states else None
+        all_self_attentions = () if output_attentions else None
+        all_cross_attentions = () if output_attentions and self.config.add_cross_attention else None
+        next_decoder_cache = () if use_cache else None
+        for i, mega_layer in enumerate(self.layers):
+            current_decoder_cache = past_key_values[i] if past_key_values is not None else None
+            mega_outputs = mega_layer(
+                hidden_states=hidden_states,
+                attention_mask=attention_mask,
+                causal_mask=causal_mask,
+                encoder_hidden_states=encoder_hidden_states,
+                encoder_attention_mask=encoder_attention_mask,
+                past_key_value=current_decoder_cache,
+                output_attentions=output_attentions,
+                use_cache=use_cache,
+            )
+
+            hidden_states = mega_outputs[0]
+            if output_hidden_states:
+                # store layer-wise hidden states in the way that the user expects
+                # (seq len X batch X embed dim) --> (batch X seq len X embed dim)
+                all_hidden_states += (hidden_states.transpose(0, 1),)
+            if output_attentions:
+                self_attn_weights = mega_outputs[1]
+                all_self_attentions += (self_attn_weights,)
+                if self.config.add_cross_attention:
+                    cross_attn_weights = mega_outputs[2]
+                    all_cross_attentions += (cross_attn_weights,)
+            if use_cache:
+                updated_cache = mega_outputs[-1]
+                next_decoder_cache += (updated_cache,)
+
+        # transpose final hidden states
+        hidden_states = hidden_states.transpose(0, 1)
+
+        # optional pooling layer
+        pooled_output = self.pooler(hidden_states) if self.pooler is not None else None
+
+        if not return_dict:
+            return (hidden_states, pooled_output) + (
+                all_hidden_states,
+                next_decoder_cache,
+                all_self_attentions,
+                all_cross_attentions,
+            )
+
+        return BaseModelOutputWithPoolingAndCrossAttentions(
+            last_hidden_state=hidden_states,
+            pooler_output=pooled_output,
+            past_key_values=next_decoder_cache,
+            hidden_states=all_hidden_states,
+            attentions=all_self_attentions,
+            cross_attentions=all_cross_attentions,
+        )
+
+
+@add_start_docstrings(
+    """MEGA Model with a `language modeling` head on top for CLM fine-tuning.""", MEGA_START_DOCSTRING
+)
+class MegaForCausalLM(MegaPreTrainedModel):
+    _tied_weights_keys = ["lm_head.weight"]
+
+    def __init__(self, config: MegaConfig):
+        super().__init__(config)
+
+        if not config.is_decoder:
+            logger.warning("If you want to use `MegaForCausalLM` as a standalone, add `is_decoder=True.`")
+
+        self.mega = MegaModel(config, add_pooling_layer=False)
+
+        if config.add_lm_hidden_dense_layer:
+            self.dense = nn.Linear(config.hidden_size, config.hidden_size)
+            self.hidden_activation = nn.Tanh()
+        else:
+            self.dense = None
+            self.hidden_activation = None
+
+        self.lm_head = nn.Linear(config.hidden_size, config.vocab_size)
+
+        # Initialize weights and apply final processing
+        self.post_init()
+
+    def get_output_embeddings(self):
+        return self.lm_head
+
+    def set_output_embeddings(self, new_embeddings):
+        self.lm_head = new_embeddings
+
+    @add_start_docstrings_to_model_forward(MEGA_INPUTS_DOCSTRING.format("batch_size, sequence_length"))
+    @replace_return_docstrings(output_type=CausalLMOutputWithCrossAttentions, config_class=_CONFIG_FOR_DOC)
+    def forward(
+        self,
+        input_ids: Optional[torch.LongTensor] = None,
+        attention_mask: Optional[torch.FloatTensor] = None,
+        token_type_ids: Optional[torch.LongTensor] = None,
+        inputs_embeds: Optional[torch.FloatTensor] = None,
+        encoder_hidden_states: Optional[torch.FloatTensor] = None,
+        encoder_attention_mask: Optional[torch.FloatTensor] = None,
+        labels: Optional[torch.LongTensor] = None,
+        past_key_values: Tuple[Tuple[torch.FloatTensor]] = None,
+        use_cache: Optional[bool] = None,
+        output_attentions: Optional[bool] = None,
+        output_hidden_states: Optional[bool] = None,
+        return_dict: Optional[bool] = None,
+    ) -> Union[Tuple[torch.Tensor], CausalLMOutputWithCrossAttentions]:
+        r"""
+        encoder_hidden_states  (`torch.FloatTensor` of shape `(batch_size, sequence_length, hidden_size)`, *optional*):
+            Sequence of hidden-states at the output of the last layer of the encoder. Used in the cross-attention if
+            the model is configured as a decoder.
+        encoder_attention_mask (`torch.FloatTensor` of shape `(batch_size, sequence_length)`, *optional*):
+            Mask to avoid performing attention on the padding token indices of the encoder input. This mask is used in
+            the cross-attention if the model is configured as a decoder. Mask values selected in `[0, 1]`:
+
+            - 1 for tokens that are **not masked**,
+            - 0 for tokens that are **masked**.
+
+        labels (`torch.LongTensor` of shape `(batch_size, sequence_length)`, *optional*):
+            Labels for computing the left-to-right language modeling loss (next word prediction). Indices should be in
+            `[-100, 0, ..., config.vocab_size]` (see `input_ids` docstring) Tokens with indices set to `-100` are
+            ignored (masked), the loss is only computed for the tokens with labels in `[0, ..., config.vocab_size]`
+        past_key_values (`tuple(tuple(torch.FloatTensor))` of length `config.n_layers` with each tuple having 4 tensors of shape `(batch_size, num_heads, sequence_length - 1, embed_size_per_head)`):
+            Contains precomputed key and value hidden states of the attention blocks. Can be used to speed up decoding.
+
+            If `past_key_values` are used, the user can optionally input only the last `decoder_input_ids` (those that
+            don't have their past key value states given to this model) of shape `(batch_size, 1)` instead of all
+            `decoder_input_ids` of shape `(batch_size, sequence_length)`.
+        use_cache (`bool`, *optional*):
+            If set to `True`, `past_key_values` key value states are returned and can be used to speed up decoding (see
+            `past_key_values`).
+
+        Returns:
+
+        Example:
+
+        ```python
+        >>> from transformers import AutoTokenizer, MegaForCausalLM, AutoConfig
+        >>> import torch
+
+        >>> tokenizer = AutoTokenizer.from_pretrained("mnaylor/mega-base-wikitext")
+        >>> config = AutoConfig.from_pretrained("mnaylor/mega-base-wikitext")
+        >>> config.is_decoder = True
+        >>> config.bidirectional = False
+        >>> model = MegaForCausalLM.from_pretrained(
+        ...     "mnaylor/mega-base-wikitext", config=config, ignore_mismatched_sizes=True
+        ... )
+
+        >>> inputs = tokenizer("Hello, my dog is cute", return_tensors="pt")
+        >>> outputs = model(**inputs)
+
+        >>> prediction_logits = outputs.logits
+        ```"""
+        return_dict = return_dict if return_dict is not None else self.config.use_return_dict
+        if labels is not None:
+            use_cache = False
+
+        outputs = self.mega(
+            input_ids,
+            attention_mask=attention_mask,
+            token_type_ids=token_type_ids,
+            inputs_embeds=inputs_embeds,
+            encoder_hidden_states=encoder_hidden_states,
+            encoder_attention_mask=encoder_attention_mask,
+            past_key_values=past_key_values,
+            use_cache=use_cache,
+            output_attentions=output_attentions,
+            output_hidden_states=output_hidden_states,
+            return_dict=return_dict,
+        )
+
+        sequence_output = outputs[0]
+
+        if self.dense is not None:
+            sequence_output = self.dense(sequence_output)
+            sequence_output = self.hidden_activation(sequence_output)
+
+        prediction_scores = self.lm_head(sequence_output)
+
+        lm_loss = None
+        if labels is not None:
+            # we are doing next-token prediction; shift prediction scores and input ids by one
+            shifted_prediction_scores = prediction_scores[:, :-1, :].contiguous()
+            labels = labels[:, 1:].contiguous()
+            loss_fct = CrossEntropyLoss()
+            lm_loss = loss_fct(shifted_prediction_scores.view(-1, self.config.vocab_size), labels.view(-1))
+
+        if not return_dict:
+            output = (prediction_scores,) + outputs[2:]
+            return ((lm_loss,) + output) if lm_loss is not None else output
+
+        return CausalLMOutputWithCrossAttentions(
+            loss=lm_loss,
+            logits=prediction_scores,
+            past_key_values=outputs.past_key_values,
+            hidden_states=outputs.hidden_states,
+            attentions=outputs.attentions,
+            cross_attentions=outputs.cross_attentions,
+        )
+
+    def prepare_inputs_for_generation(self, input_ids, past_key_values=None, attention_mask=None, **model_kwargs):
+        input_shape = input_ids.shape
+        # if model is used as a decoder in encoder-decoder model, the decoder attention mask is created on the fly
+        if attention_mask is None:
+            attention_mask = input_ids.new_ones(input_shape)
+
+        # cut decoder_input_ids if past is used
+        if past_key_values is not None:
+            input_ids = input_ids[:, -1:]
+
+        return {"input_ids": input_ids, "attention_mask": attention_mask, "past_key_values": past_key_values}
+
+    def _reorder_cache(self, past_key_values, beam_idx):
+        reordered_past = ()
+        for layer_past in past_key_values:
+            reordered_past += (
+                tuple(past_state.index_select(0, beam_idx.to(past_state.device)) for past_state in layer_past),
+            )
+        return reordered_past
+
+
+@add_start_docstrings("""MEGA Model with a `language modeling` head on top.""", MEGA_START_DOCSTRING)
+class MegaForMaskedLM(MegaPreTrainedModel):
+    _tied_weights_keys = ["mlm_head.weight"]
+
+    def __init__(self, config: MegaConfig):
+        super().__init__(config)
+
+        if config.is_decoder:
+            logger.warning(
+                "If you want to use `MegaForMaskedLM`, set `config.is_decoder=False` for "
+                "bi-directional self-attention."
+            )
+
+        self.mega = MegaModel(config, add_pooling_layer=False)
+        if config.add_lm_hidden_dense_layer:
+            self.dense = nn.Linear(config.hidden_size, config.hidden_size)
+            self.hidden_activation = nn.Tanh()
+        else:
+            self.dense = None
+            self.hidden_activation = None
+        self.mlm_head = nn.Linear(config.hidden_size, config.vocab_size)
+        self.dropout = nn.Dropout(config.dropout_prob)
+
+        # Initialize weights and apply final processing
+        self.post_init()
+
+    def get_output_embeddings(self):
+        return self.mlm_head
+
+    def set_output_embeddings(self, new_embeddings):
+        self.mlm_head = new_embeddings
+
+    @add_start_docstrings_to_model_forward(MEGA_INPUTS_DOCSTRING.format("batch_size, sequence_length"))
+    @add_code_sample_docstrings(
+        checkpoint=_CHECKPOINT_FOR_DOC,
+        output_type=MaskedLMOutput,
+        config_class=_CONFIG_FOR_DOC,
+        mask="<mask>",
+        expected_output="' Paris'",
+        expected_loss=0.1,
+    )
+    def forward(
+        self,
+        input_ids: Optional[torch.LongTensor] = None,
+        attention_mask: Optional[torch.FloatTensor] = None,
+        token_type_ids: Optional[torch.LongTensor] = None,
+        inputs_embeds: Optional[torch.FloatTensor] = None,
+        encoder_hidden_states: Optional[torch.FloatTensor] = None,
+        encoder_attention_mask: Optional[torch.FloatTensor] = None,
+        labels: Optional[torch.LongTensor] = None,
+        output_attentions: Optional[bool] = None,
+        output_hidden_states: Optional[bool] = None,
+        return_dict: Optional[bool] = None,
+    ) -> Union[Tuple[torch.Tensor], MaskedLMOutput]:
+        r"""
+        labels (`torch.LongTensor` of shape `(batch_size, sequence_length)`, *optional*):
+            Labels for computing the masked language modeling loss. Indices should be in `[-100, 0, ...,
+            config.vocab_size]` (see `input_ids` docstring) Tokens with indices set to `-100` are ignored (masked), the
+            loss is only computed for the tokens with labels in `[0, ..., config.vocab_size]`
+        kwargs (`Dict[str, any]`, optional, defaults to *{}*):
+            Used to hide legacy arguments that have been deprecated.
+        """
+        return_dict = return_dict if return_dict is not None else self.config.use_return_dict
+
+        outputs = self.mega(
+            input_ids,
+            attention_mask=attention_mask,
+            token_type_ids=token_type_ids,
+            inputs_embeds=inputs_embeds,
+            encoder_hidden_states=encoder_hidden_states,
+            encoder_attention_mask=encoder_attention_mask,
+            output_attentions=output_attentions,
+            output_hidden_states=output_hidden_states,
+            return_dict=return_dict,
+        )
+        sequence_output = outputs[0]
+        if self.dense is not None:
+            sequence_output = self.dense(sequence_output)
+            sequence_output = self.hidden_activation(sequence_output)
+        prediction_scores = self.mlm_head(sequence_output)
+
+        masked_lm_loss = None
+        if labels is not None:
+            loss_fct = CrossEntropyLoss()
+            masked_lm_loss = loss_fct(prediction_scores.view(-1, self.config.vocab_size), labels.view(-1))
+
+        if not return_dict:
+            output = (prediction_scores,) + outputs[2:]
+            return ((masked_lm_loss,) + output) if masked_lm_loss is not None else output
+
+        return MaskedLMOutput(
+            loss=masked_lm_loss,
+            logits=prediction_scores,
+            hidden_states=outputs.hidden_states,
+            attentions=outputs.attentions,
+        )
+
+
+@add_start_docstrings(
+    """
+    MEGA Model transformer with a sequence classification/regression head on top (a linear layer on top of the pooled
+    output) e.g. for GLUE tasks.
+    """,
+    MEGA_START_DOCSTRING,
+)
+class MegaForSequenceClassification(MegaPreTrainedModel):
+    def __init__(self, config):
+        super().__init__(config)
+        self.num_labels = config.num_labels
+        self.config = config
+
+        self.mega = MegaModel(config, add_pooling_layer=False)
+        self.classifier = MegaClassificationHead(config)
+
+        # Initialize weights and apply final processing
+        self.post_init()
+
+    @add_start_docstrings_to_model_forward(MEGA_INPUTS_DOCSTRING.format("batch_size, sequence_length"))
+    @add_code_sample_docstrings(
+        checkpoint=_CHECKPOINT_FOR_DOC,
+        output_type=SequenceClassifierOutput,
+        config_class=_CONFIG_FOR_DOC,
+    )
+    def forward(
+        self,
+        input_ids: Optional[torch.LongTensor] = None,
+        attention_mask: Optional[torch.FloatTensor] = None,
+        token_type_ids: Optional[torch.LongTensor] = None,
+        inputs_embeds: Optional[torch.FloatTensor] = None,
+        labels: Optional[torch.LongTensor] = None,
+        output_attentions: Optional[bool] = None,
+        output_hidden_states: Optional[bool] = None,
+        return_dict: Optional[bool] = None,
+    ) -> Union[Tuple[torch.Tensor], SequenceClassifierOutput]:
+        r"""
+        labels (`torch.LongTensor` of shape `(batch_size,)`, *optional*):
+            Labels for computing the sequence classification/regression loss. Indices should be in `[0, ...,
+            config.num_labels - 1]`. If `config.num_labels == 1` a regression loss is computed (Mean-Square loss), If
+            `config.num_labels > 1` a classification loss is computed (Cross-Entropy).
+        """
+        return_dict = return_dict if return_dict is not None else self.config.use_return_dict
+
+        outputs = self.mega(
+            input_ids,
+            attention_mask=attention_mask,
+            token_type_ids=token_type_ids,
+            inputs_embeds=inputs_embeds,
+            output_attentions=output_attentions,
+            output_hidden_states=output_hidden_states,
+            return_dict=return_dict,
+        )
+        sequence_output = outputs[0]
+        logits = self.classifier(sequence_output)
+
+        loss = None
+        if labels is not None:
+            if self.config.problem_type is None:
+                if self.num_labels == 1:
+                    self.config.problem_type = "regression"
+                elif self.num_labels > 1 and (labels.dtype == torch.long or labels.dtype == torch.int):
+                    self.config.problem_type = "single_label_classification"
+                else:
+                    self.config.problem_type = "multi_label_classification"
+
+            if self.config.problem_type == "regression":
+                loss_fct = MSELoss()
+                if self.num_labels == 1:
+                    loss = loss_fct(logits.squeeze(), labels.squeeze())
+                else:
+                    loss = loss_fct(logits, labels)
+            elif self.config.problem_type == "single_label_classification":
+                loss_fct = CrossEntropyLoss()
+                loss = loss_fct(logits.view(-1, self.num_labels), labels.view(-1))
+            elif self.config.problem_type == "multi_label_classification":
+                loss_fct = BCEWithLogitsLoss()
+                loss = loss_fct(logits, labels)
+
+        if not return_dict:
+            output = (logits,) + outputs[2:]
+            return ((loss,) + output) if loss is not None else output
+
+        return SequenceClassifierOutput(
+            loss=loss,
+            logits=logits,
+            hidden_states=outputs.hidden_states,
+            attentions=outputs.attentions,
+        )
+
+
+@add_start_docstrings(
+    """
+    MEGA Model with a multiple choice classification head on top (a linear layer on top of the pooled output and a
+    softmax) e.g. for RocStories/SWAG tasks.
+    """,
+    MEGA_START_DOCSTRING,
+)
+class MegaForMultipleChoice(MegaPreTrainedModel):
+    def __init__(self, config):
+        super().__init__(config)
+
+        self.mega = MegaModel(config)
+        self.dropout = nn.Dropout(config.hidden_dropout_prob)
+        self.classifier = nn.Linear(config.hidden_size, 1)
+
+        # Initialize weights and apply final processing
+        self.post_init()
+
+    @add_start_docstrings_to_model_forward(MEGA_INPUTS_DOCSTRING.format("batch_size, num_choices, sequence_length"))
+    @add_code_sample_docstrings(
+        checkpoint=_CHECKPOINT_FOR_DOC,
+        output_type=MultipleChoiceModelOutput,
+        config_class=_CONFIG_FOR_DOC,
+    )
+    def forward(
+        self,
+        input_ids: Optional[torch.LongTensor] = None,
+        token_type_ids: Optional[torch.LongTensor] = None,
+        attention_mask: Optional[torch.FloatTensor] = None,
+        labels: Optional[torch.LongTensor] = None,
+        inputs_embeds: Optional[torch.FloatTensor] = None,
+        output_attentions: Optional[bool] = None,
+        output_hidden_states: Optional[bool] = None,
+        return_dict: Optional[bool] = None,
+    ) -> Union[Tuple[torch.Tensor], MultipleChoiceModelOutput]:
+        r"""
+        labels (`torch.LongTensor` of shape `(batch_size,)`, *optional*):
+            Labels for computing the multiple choice classification loss. Indices should be in `[0, ...,
+            num_choices-1]` where `num_choices` is the size of the second dimension of the input tensors. (See
+            `input_ids` above)
+        """
+        return_dict = return_dict if return_dict is not None else self.config.use_return_dict
+        num_choices = input_ids.shape[1] if input_ids is not None else inputs_embeds.shape[1]
+
+        flat_input_ids = input_ids.view(-1, input_ids.size(-1)) if input_ids is not None else None
+        flat_token_type_ids = token_type_ids.view(-1, token_type_ids.size(-1)) if token_type_ids is not None else None
+        flat_attention_mask = attention_mask.view(-1, attention_mask.size(-1)) if attention_mask is not None else None
+        flat_inputs_embeds = (
+            inputs_embeds.view(-1, inputs_embeds.size(-2), inputs_embeds.size(-1))
+            if inputs_embeds is not None
+            else None
+        )
+
+        outputs = self.mega(
+            flat_input_ids,
+            token_type_ids=flat_token_type_ids,
+            attention_mask=flat_attention_mask,
+            inputs_embeds=flat_inputs_embeds,
+            output_attentions=output_attentions,
+            output_hidden_states=output_hidden_states,
+            return_dict=return_dict,
+        )
+        pooled_output = outputs[1]
+
+        pooled_output = self.dropout(pooled_output)
+        logits = self.classifier(pooled_output)
+        reshaped_logits = logits.view(-1, num_choices)
+
+        loss = None
+        if labels is not None:
+            loss_fct = CrossEntropyLoss()
+            loss = loss_fct(reshaped_logits, labels)
+
+        if not return_dict:
+            output = (reshaped_logits,) + outputs[2:]
+            return ((loss,) + output) if loss is not None else output
+
+        return MultipleChoiceModelOutput(
+            loss=loss,
+            logits=reshaped_logits,
+            hidden_states=outputs.hidden_states,
+            attentions=outputs.attentions,
+        )
+
+
+@add_start_docstrings(
+    """
+    MEGA Model with a token classification head on top (a linear layer on top of the hidden-states output) e.g. for
+    Named-Entity-Recognition (NER) tasks.
+    """,
+    MEGA_START_DOCSTRING,
+)
+class MegaForTokenClassification(MegaPreTrainedModel):
+    def __init__(self, config):
+        super().__init__(config)
+        self.num_labels = config.num_labels
+
+        self.mega = MegaModel(config, add_pooling_layer=False)
+        classifier_dropout = (
+            config.classifier_dropout if config.classifier_dropout is not None else config.hidden_dropout_prob
+        )
+        self.dropout = nn.Dropout(classifier_dropout)
+        self.classifier = nn.Linear(config.hidden_size, config.num_labels)
+
+        # Initialize weights and apply final processing
+        self.post_init()
+
+    @add_start_docstrings_to_model_forward(MEGA_INPUTS_DOCSTRING.format("batch_size, sequence_length"))
+    @add_code_sample_docstrings(
+        checkpoint=_CHECKPOINT_FOR_DOC,
+        output_type=TokenClassifierOutput,
+        config_class=_CONFIG_FOR_DOC,
+    )
+    def forward(
+        self,
+        input_ids: Optional[torch.LongTensor] = None,
+        attention_mask: Optional[torch.FloatTensor] = None,
+        token_type_ids: Optional[torch.LongTensor] = None,
+        inputs_embeds: Optional[torch.FloatTensor] = None,
+        labels: Optional[torch.LongTensor] = None,
+        output_attentions: Optional[bool] = None,
+        output_hidden_states: Optional[bool] = None,
+        return_dict: Optional[bool] = None,
+    ) -> Union[Tuple[torch.Tensor], TokenClassifierOutput]:
+        r"""
+        labels (`torch.LongTensor` of shape `(batch_size, sequence_length)`, *optional*):
+            Labels for computing the token classification loss. Indices should be in `[0, ..., config.num_labels - 1]`.
+        """
+        return_dict = return_dict if return_dict is not None else self.config.use_return_dict
+
+        outputs = self.mega(
+            input_ids,
+            attention_mask=attention_mask,
+            token_type_ids=token_type_ids,
+            inputs_embeds=inputs_embeds,
+            output_attentions=output_attentions,
+            output_hidden_states=output_hidden_states,
+            return_dict=return_dict,
+        )
+
+        sequence_output = outputs[0]
+
+        sequence_output = self.dropout(sequence_output)
+        logits = self.classifier(sequence_output)
+
+        loss = None
+        if labels is not None:
+            loss_fct = CrossEntropyLoss()
+            loss = loss_fct(logits.view(-1, self.num_labels), labels.view(-1))
+
+        if not return_dict:
+            output = (logits,) + outputs[2:]
+            return ((loss,) + output) if loss is not None else output
+
+        return TokenClassifierOutput(
+            loss=loss,
+            logits=logits,
+            hidden_states=outputs.hidden_states,
+            attentions=outputs.attentions,
+        )
+
+
+# copied from transformers.models.roberta.modeling_roberta.RobertaClassificationHead with Roberta->Mega
+class MegaClassificationHead(nn.Module):
+    """Head for sentence-level classification tasks."""
+
+    def __init__(self, config):
+        super().__init__()
+        self.dense = nn.Linear(config.hidden_size, config.hidden_size)
+        classifier_dropout = (
+            config.classifier_dropout if config.classifier_dropout is not None else config.hidden_dropout_prob
+        )
+        self.dropout = nn.Dropout(classifier_dropout)
+        self.out_proj = nn.Linear(config.hidden_size, config.num_labels)
+
+    def forward(self, features, **kwargs):
+        x = features[:, 0, :]  # take <s> token (equiv. to [CLS])
+        x = self.dropout(x)
+        x = self.dense(x)
+        x = torch.tanh(x)
+        x = self.dropout(x)
+        x = self.out_proj(x)
+        return x
+
+
+@add_start_docstrings(
+    """
+    MEGA Model with a span classification head on top for extractive question-answering tasks like SQuAD (a linear
+    layers on top of the hidden-states output to compute `span start logits` and `span end logits`).
+    """,
+    MEGA_START_DOCSTRING,
+)
+class MegaForQuestionAnswering(MegaPreTrainedModel):
+    def __init__(self, config):
+        super().__init__(config)
+        self.num_labels = config.num_labels
+
+        self.mega = MegaModel(config, add_pooling_layer=False)
+        self.qa_outputs = nn.Linear(config.hidden_size, config.num_labels)
+
+        # Initialize weights and apply final processing
+        self.post_init()
+
+    @add_start_docstrings_to_model_forward(MEGA_INPUTS_DOCSTRING.format("batch_size, sequence_length"))
+    @add_code_sample_docstrings(
+        checkpoint=_CHECKPOINT_FOR_DOC,
+        output_type=QuestionAnsweringModelOutput,
+        config_class=_CONFIG_FOR_DOC,
+    )
+    def forward(
+        self,
+        input_ids: Optional[torch.LongTensor] = None,
+        attention_mask: Optional[torch.FloatTensor] = None,
+        token_type_ids: Optional[torch.LongTensor] = None,
+        inputs_embeds: Optional[torch.FloatTensor] = None,
+        start_positions: Optional[torch.LongTensor] = None,
+        end_positions: Optional[torch.LongTensor] = None,
+        output_attentions: Optional[bool] = None,
+        output_hidden_states: Optional[bool] = None,
+        return_dict: Optional[bool] = None,
+    ) -> Union[Tuple[torch.Tensor], QuestionAnsweringModelOutput]:
+        r"""
+        start_positions (`torch.LongTensor` of shape `(batch_size,)`, *optional*):
+            Labels for position (index) of the start of the labelled span for computing the token classification loss.
+            Positions are clamped to the length of the sequence (`sequence_length`). Position outside of the sequence
+            are not taken into account for computing the loss.
+        end_positions (`torch.LongTensor` of shape `(batch_size,)`, *optional*):
+            Labels for position (index) of the end of the labelled span for computing the token classification loss.
+            Positions are clamped to the length of the sequence (`sequence_length`). Position outside of the sequence
+            are not taken into account for computing the loss.
+        """
+        return_dict = return_dict if return_dict is not None else self.config.use_return_dict
+
+        outputs = self.mega(
+            input_ids,
+            attention_mask=attention_mask,
+            token_type_ids=token_type_ids,
+            inputs_embeds=inputs_embeds,
+            output_attentions=output_attentions,
+            output_hidden_states=output_hidden_states,
+            return_dict=return_dict,
+        )
+
+        sequence_output = outputs[0]
+
+        logits = self.qa_outputs(sequence_output)
+        start_logits, end_logits = logits.split(1, dim=-1)
+        start_logits = start_logits.squeeze(-1).contiguous()
+        end_logits = end_logits.squeeze(-1).contiguous()
+
+        total_loss = None
+        if start_positions is not None and end_positions is not None:
+            # If we are on multi-GPU, split add a dimension
+            if len(start_positions.size()) > 1:
+                start_positions = start_positions.squeeze(-1)
+            if len(end_positions.size()) > 1:
+                end_positions = end_positions.squeeze(-1)
+            # sometimes the start/end positions are outside our model inputs, we ignore these terms
+            ignored_index = start_logits.size(1)
+            start_positions = start_positions.clamp(0, ignored_index)
+            end_positions = end_positions.clamp(0, ignored_index)
+
+            loss_fct = CrossEntropyLoss(ignore_index=ignored_index)
+            start_loss = loss_fct(start_logits, start_positions)
+            end_loss = loss_fct(end_logits, end_positions)
+            total_loss = (start_loss + end_loss) / 2
+
+        if not return_dict:
+            output = (start_logits, end_logits) + outputs[2:]
+            return ((total_loss,) + output) if total_loss is not None else output
+
+        return QuestionAnsweringModelOutput(
+            loss=total_loss,
+            start_logits=start_logits,
+            end_logits=end_logits,
+            hidden_states=outputs.hidden_states,
+            attentions=outputs.attentions,
+        )