core/modules/x_transformer.py

from functools import partial
from inspect import isfunction
from collections import namedtuple
from einops import rearrange, repeat

import torch
from torch import nn, einsum
import torch.nn.functional as F

DEFAULT_DIM_HEAD = 64

Intermediates = namedtuple("Intermediates", ["pre_softmax_attn", "post_softmax_attn"])

LayerIntermediates = namedtuple("Intermediates", ["hiddens", "attn_intermediates"])


class AbsolutePositionalEmbedding(nn.Module):
    def __init__(self, dim, max_seq_len):
        super().__init__()
        self.emb = nn.Embedding(max_seq_len, dim)
        self.init_()

    def init_(self):
        nn.init.normal_(self.emb.weight, std=0.02)

    def forward(self, x):
        n = torch.arange(x.shape[1], device=x.device)
        return self.emb(n)[None, :, :]


class FixedPositionalEmbedding(nn.Module):
    def __init__(self, dim):
        super().__init__()
        inv_freq = 1.0 / (10000 ** (torch.arange(0, dim, 2).float() / dim))
        self.register_buffer("inv_freq", inv_freq)

    def forward(self, x, seq_dim=1, offset=0):
        t = (
            torch.arange(x.shape[seq_dim], device=x.device).type_as(self.inv_freq)
            + offset
        )
        sinusoid_inp = torch.einsum("i , j -> i j", t, self.inv_freq)
        emb = torch.cat((sinusoid_inp.sin(), sinusoid_inp.cos()), dim=-1)
        return emb[None, :, :]


def exists(val):
    return val is not None


def default(val, d):
    if exists(val):
        return val
    return d() if isfunction(d) else d


def always(val):
    def inner(*args, **kwargs):
        return val

    return inner


def not_equals(val):
    def inner(x):
        return x != val

    return inner


def equals(val):
    def inner(x):
        return x == val

    return inner


def max_neg_value(tensor):
    return -torch.finfo(tensor.dtype).max


def pick_and_pop(keys, d):
    values = list(map(lambda key: d.pop(key), keys))
    return dict(zip(keys, values))


def group_dict_by_key(cond, d):
    return_val = [dict(), dict()]
    for key in d.keys():
        match = bool(cond(key))
        ind = int(not match)
        return_val[ind][key] = d[key]
    return (*return_val,)


def string_begins_with(prefix, str):
    return str.startswith(prefix)


def group_by_key_prefix(prefix, d):
    return group_dict_by_key(partial(string_begins_with, prefix), d)


def groupby_prefix_and_trim(prefix, d):
    kwargs_with_prefix, kwargs = group_dict_by_key(
        partial(string_begins_with, prefix), d
    )
    kwargs_without_prefix = dict(
        map(lambda x: (x[0][len(prefix) :], x[1]), tuple(kwargs_with_prefix.items()))
    )
    return kwargs_without_prefix, kwargs


class Scale(nn.Module):
    def __init__(self, value, fn):
        super().__init__()
        self.value = value
        self.fn = fn

    def forward(self, x, **kwargs):
        x, *rest = self.fn(x, **kwargs)
        return (x * self.value, *rest)


class Rezero(nn.Module):
    def __init__(self, fn):
        super().__init__()
        self.fn = fn
        self.g = nn.Parameter(torch.zeros(1))

    def forward(self, x, **kwargs):
        x, *rest = self.fn(x, **kwargs)
        return (x * self.g, *rest)


class ScaleNorm(nn.Module):
    def __init__(self, dim, eps=1e-5):
        super().__init__()
        self.scale = dim**-0.5
        self.eps = eps
        self.g = nn.Parameter(torch.ones(1))

    def forward(self, x):
        norm = torch.norm(x, dim=-1, keepdim=True) * self.scale
        return x / norm.clamp(min=self.eps) * self.g


class RMSNorm(nn.Module):
    def __init__(self, dim, eps=1e-8):
        super().__init__()
        self.scale = dim**-0.5
        self.eps = eps
        self.g = nn.Parameter(torch.ones(dim))

    def forward(self, x):
        norm = torch.norm(x, dim=-1, keepdim=True) * self.scale
        return x / norm.clamp(min=self.eps) * self.g


class Residual(nn.Module):
    def forward(self, x, residual):
        return x + residual


class GRUGating(nn.Module):
    def __init__(self, dim):
        super().__init__()
        self.gru = nn.GRUCell(dim, dim)

    def forward(self, x, residual):
        gated_output = self.gru(
            rearrange(x, "b n d -> (b n) d"), rearrange(residual, "b n d -> (b n) d")
        )

        return gated_output.reshape_as(x)


class GEGLU(nn.Module):
    def __init__(self, dim_in, dim_out):
        super().__init__()
        self.proj = nn.Linear(dim_in, dim_out * 2)

    def forward(self, x):
        x, gate = self.proj(x).chunk(2, dim=-1)
        return x * F.gelu(gate)


class FeedForward(nn.Module):
    def __init__(self, dim, dim_out=None, mult=4, glu=False, dropout=0.0):
        super().__init__()
        inner_dim = int(dim * mult)
        dim_out = default(dim_out, dim)
        project_in = (
            nn.Sequential(nn.Linear(dim, inner_dim), nn.GELU())
            if not glu
            else GEGLU(dim, inner_dim)
        )

        self.net = nn.Sequential(
            project_in, nn.Dropout(dropout), nn.Linear(inner_dim, dim_out)
        )

    def forward(self, x):
        return self.net(x)


class Attention(nn.Module):
    def __init__(
        self,
        dim,
        dim_head=DEFAULT_DIM_HEAD,
        heads=8,
        causal=False,
        mask=None,
        talking_heads=False,
        sparse_topk=None,
        use_entmax15=False,
        num_mem_kv=0,
        dropout=0.0,
        on_attn=False,
    ):
        super().__init__()
        if use_entmax15:
            raise NotImplementedError(
                "Check out entmax activation instead of softmax activation!"
            )
        self.scale = dim_head**-0.5
        self.heads = heads
        self.causal = causal
        self.mask = mask

        inner_dim = dim_head * heads

        self.to_q = nn.Linear(dim, inner_dim, bias=False)
        self.to_k = nn.Linear(dim, inner_dim, bias=False)
        self.to_v = nn.Linear(dim, inner_dim, bias=False)
        self.dropout = nn.Dropout(dropout)

        self.talking_heads = talking_heads
        if talking_heads:
            self.pre_softmax_proj = nn.Parameter(torch.randn(heads, heads))
            self.post_softmax_proj = nn.Parameter(torch.randn(heads, heads))

        self.sparse_topk = sparse_topk
        self.attn_fn = F.softmax

        self.num_mem_kv = num_mem_kv
        if num_mem_kv > 0:
            self.mem_k = nn.Parameter(torch.randn(heads, num_mem_kv, dim_head))
            self.mem_v = nn.Parameter(torch.randn(heads, num_mem_kv, dim_head))

        self.attn_on_attn = on_attn
        self.to_out = (
            nn.Sequential(nn.Linear(inner_dim, dim * 2), nn.GLU())
            if on_attn
            else nn.Linear(inner_dim, dim)
        )

    def forward(
        self,
        x,
        context=None,
        mask=None,
        context_mask=None,
        rel_pos=None,
        sinusoidal_emb=None,
        prev_attn=None,
        mem=None,
    ):
        b, n, _, h, talking_heads, device = (
            *x.shape,
            self.heads,
            self.talking_heads,
            x.device,
        )
        kv_input = default(context, x)

        q_input = x
        k_input = kv_input
        v_input = kv_input

        if exists(mem):
            k_input = torch.cat((mem, k_input), dim=-2)
            v_input = torch.cat((mem, v_input), dim=-2)

        if exists(sinusoidal_emb):
            offset = k_input.shape[-2] - q_input.shape[-2]
            q_input = q_input + sinusoidal_emb(q_input, offset=offset)
            k_input = k_input + sinusoidal_emb(k_input)

        q = self.to_q(q_input)
        k = self.to_k(k_input)
        v = self.to_v(v_input)

        q, k, v = map(lambda t: rearrange(t, "b n (h d) -> b h n d", h=h), (q, k, v))

        input_mask = None
        if any(map(exists, (mask, context_mask))):
            q_mask = default(mask, lambda: torch.ones((b, n), device=device).bool())
            k_mask = q_mask if not exists(context) else context_mask
            k_mask = default(
                k_mask, lambda: torch.ones((b, k.shape[-2]), device=device).bool()
            )
            q_mask = rearrange(q_mask, "b i -> b () i ()")
            k_mask = rearrange(k_mask, "b j -> b () () j")
            input_mask = q_mask * k_mask

        if self.num_mem_kv > 0:
            mem_k, mem_v = map(
                lambda t: repeat(t, "h n d -> b h n d", b=b), (self.mem_k, self.mem_v)
            )
            k = torch.cat((mem_k, k), dim=-2)
            v = torch.cat((mem_v, v), dim=-2)
            if exists(input_mask):
                input_mask = F.pad(input_mask, (self.num_mem_kv, 0), value=True)

        dots = einsum("b h i d, b h j d -> b h i j", q, k) * self.scale
        mask_value = max_neg_value(dots)

        if exists(prev_attn):
            dots = dots + prev_attn

        pre_softmax_attn = dots

        if talking_heads:
            dots = einsum(
                "b h i j, h k -> b k i j", dots, self.pre_softmax_proj
            ).contiguous()

        if exists(rel_pos):
            dots = rel_pos(dots)

        if exists(input_mask):
            dots.masked_fill_(~input_mask, mask_value)
            del input_mask

        if self.causal:
            i, j = dots.shape[-2:]
            r = torch.arange(i, device=device)
            mask = rearrange(r, "i -> () () i ()") < rearrange(r, "j -> () () () j")
            mask = F.pad(mask, (j - i, 0), value=False)
            dots.masked_fill_(mask, mask_value)
            del mask

        if exists(self.sparse_topk) and self.sparse_topk < dots.shape[-1]:
            top, _ = dots.topk(self.sparse_topk, dim=-1)
            vk = top[..., -1].unsqueeze(-1).expand_as(dots)
            mask = dots < vk
            dots.masked_fill_(mask, mask_value)
            del mask

        attn = self.attn_fn(dots, dim=-1)
        post_softmax_attn = attn

        attn = self.dropout(attn)

        if talking_heads:
            attn = einsum(
                "b h i j, h k -> b k i j", attn, self.post_softmax_proj
            ).contiguous()

        out = einsum("b h i j, b h j d -> b h i d", attn, v)
        out = rearrange(out, "b h n d -> b n (h d)")

        intermediates = Intermediates(
            pre_softmax_attn=pre_softmax_attn, post_softmax_attn=post_softmax_attn
        )

        return self.to_out(out), intermediates


class AttentionLayers(nn.Module):
    def __init__(
        self,
        dim,
        depth,
        heads=8,
        causal=False,
        cross_attend=False,
        only_cross=False,
        use_scalenorm=False,
        use_rmsnorm=False,
        use_rezero=False,
        rel_pos_num_buckets=32,
        rel_pos_max_distance=128,
        position_infused_attn=False,
        custom_layers=None,
        sandwich_coef=None,
        par_ratio=None,
        residual_attn=False,
        cross_residual_attn=False,
        macaron=False,
        pre_norm=True,
        gate_residual=False,
        **kwargs,
    ):
        super().__init__()
        ff_kwargs, kwargs = groupby_prefix_and_trim("ff_", kwargs)
        attn_kwargs, _ = groupby_prefix_and_trim("attn_", kwargs)

        dim_head = attn_kwargs.get("dim_head", DEFAULT_DIM_HEAD)

        self.dim = dim
        self.depth = depth
        self.layers = nn.ModuleList([])

        self.has_pos_emb = position_infused_attn
        self.pia_pos_emb = (
            FixedPositionalEmbedding(dim) if position_infused_attn else None
        )
        self.rotary_pos_emb = always(None)

        assert (
            rel_pos_num_buckets <= rel_pos_max_distance
        ), "number of relative position buckets must be less than the relative position max distance"
        self.rel_pos = None

        self.pre_norm = pre_norm

        self.residual_attn = residual_attn
        self.cross_residual_attn = cross_residual_attn

        norm_class = ScaleNorm if use_scalenorm else nn.LayerNorm
        norm_class = RMSNorm if use_rmsnorm else norm_class
        norm_fn = partial(norm_class, dim)

        norm_fn = nn.Identity if use_rezero else norm_fn
        branch_fn = Rezero if use_rezero else None

        if cross_attend and not only_cross:
            default_block = ("a", "c", "f")
        elif cross_attend and only_cross:
            default_block = ("c", "f")
        else:
            default_block = ("a", "f")

        if macaron:
            default_block = ("f",) + default_block

        if exists(custom_layers):
            layer_types = custom_layers
        elif exists(par_ratio):
            par_depth = depth * len(default_block)
            assert 1 < par_ratio <= par_depth, "par ratio out of range"
            default_block = tuple(filter(not_equals("f"), default_block))
            par_attn = par_depth // par_ratio
            depth_cut = par_depth * 2 // 3
            par_width = (depth_cut + depth_cut // par_attn) // par_attn
            assert (
                len(default_block) <= par_width
            ), "default block is too large for par_ratio"
            par_block = default_block + ("f",) * (par_width - len(default_block))
            par_head = par_block * par_attn
            layer_types = par_head + ("f",) * (par_depth - len(par_head))
        elif exists(sandwich_coef):
            assert (
                sandwich_coef > 0 and sandwich_coef <= depth
            ), "sandwich coefficient should be less than the depth"
            layer_types = (
                ("a",) * sandwich_coef
                + default_block * (depth - sandwich_coef)
                + ("f",) * sandwich_coef
            )
        else:
            layer_types = default_block * depth

        self.layer_types = layer_types
        self.num_attn_layers = len(list(filter(equals("a"), layer_types)))

        for layer_type in self.layer_types:
            if layer_type == "a":
                layer = Attention(dim, heads=heads, causal=causal, **attn_kwargs)
            elif layer_type == "c":
                layer = Attention(dim, heads=heads, **attn_kwargs)
            elif layer_type == "f":
                layer = FeedForward(dim, **ff_kwargs)
                layer = layer if not macaron else Scale(0.5, layer)
            else:
                raise Exception(f"invalid layer type {layer_type}")

            if isinstance(layer, Attention) and exists(branch_fn):
                layer = branch_fn(layer)

            if gate_residual:
                residual_fn = GRUGating(dim)
            else:
                residual_fn = Residual()

            self.layers.append(nn.ModuleList([norm_fn(), layer, residual_fn]))

    def forward(
        self,
        x,
        context=None,
        mask=None,
        context_mask=None,
        mems=None,
        return_hiddens=False,
    ):
        hiddens = []
        intermediates = []
        prev_attn = None
        prev_cross_attn = None

        mems = mems.copy() if exists(mems) else [None] * self.num_attn_layers

        for ind, (layer_type, (norm, block, residual_fn)) in enumerate(
            zip(self.layer_types, self.layers)
        ):
            is_last = ind == (len(self.layers) - 1)

            if layer_type == "a":
                hiddens.append(x)
                layer_mem = mems.pop(0)

            residual = x

            if self.pre_norm:
                x = norm(x)

            if layer_type == "a":
                out, inter = block(
                    x,
                    mask=mask,
                    sinusoidal_emb=self.pia_pos_emb,
                    rel_pos=self.rel_pos,
                    prev_attn=prev_attn,
                    mem=layer_mem,
                )
            elif layer_type == "c":
                out, inter = block(
                    x,
                    context=context,
                    mask=mask,
                    context_mask=context_mask,
                    prev_attn=prev_cross_attn,
                )
            elif layer_type == "f":
                out = block(x)

            x = residual_fn(out, residual)

            if layer_type in ("a", "c"):
                intermediates.append(inter)

            if layer_type == "a" and self.residual_attn:
                prev_attn = inter.pre_softmax_attn
            elif layer_type == "c" and self.cross_residual_attn:
                prev_cross_attn = inter.pre_softmax_attn

            if not self.pre_norm and not is_last:
                x = norm(x)

        if return_hiddens:
            intermediates = LayerIntermediates(
                hiddens=hiddens, attn_intermediates=intermediates
            )

            return x, intermediates

        return x


class Encoder(AttentionLayers):
    def __init__(self, **kwargs):
        assert "causal" not in kwargs, "cannot set causality on encoder"
        super().__init__(causal=False, **kwargs)


class TransformerWrapper(nn.Module):
    def __init__(
        self,
        *,
        num_tokens,
        max_seq_len,
        attn_layers,
        emb_dim=None,
        max_mem_len=0.0,
        emb_dropout=0.0,
        num_memory_tokens=None,
        tie_embedding=False,
        use_pos_emb=True,
    ):
        super().__init__()
        assert isinstance(
            attn_layers, AttentionLayers
        ), "attention layers must be one of Encoder or Decoder"

        dim = attn_layers.dim
        emb_dim = default(emb_dim, dim)

        self.max_seq_len = max_seq_len
        self.max_mem_len = max_mem_len
        self.num_tokens = num_tokens

        self.token_emb = nn.Embedding(num_tokens, emb_dim)
        self.pos_emb = (
            AbsolutePositionalEmbedding(emb_dim, max_seq_len)
            if (use_pos_emb and not attn_layers.has_pos_emb)
            else always(0)
        )
        self.emb_dropout = nn.Dropout(emb_dropout)

        self.project_emb = nn.Linear(emb_dim, dim) if emb_dim != dim else nn.Identity()
        self.attn_layers = attn_layers
        self.norm = nn.LayerNorm(dim)

        self.init_()

        self.to_logits = (
            nn.Linear(dim, num_tokens)
            if not tie_embedding
            else lambda t: t @ self.token_emb.weight.t()
        )

        num_memory_tokens = default(num_memory_tokens, 0)
        self.num_memory_tokens = num_memory_tokens
        if num_memory_tokens > 0:
            self.memory_tokens = nn.Parameter(torch.randn(num_memory_tokens, dim))

            if hasattr(attn_layers, "num_memory_tokens"):
                attn_layers.num_memory_tokens = num_memory_tokens

    def init_(self):
        nn.init.normal_(self.token_emb.weight, std=0.02)

    def forward(
        self,
        x,
        return_embeddings=False,
        mask=None,
        return_mems=False,
        return_attn=False,
        mems=None,
        **kwargs,
    ):
        b, n, device, num_mem = *x.shape, x.device, self.num_memory_tokens
        x = self.token_emb(x)
        x += self.pos_emb(x)
        x = self.emb_dropout(x)

        x = self.project_emb(x)

        if num_mem > 0:
            mem = repeat(self.memory_tokens, "n d -> b n d", b=b)
            x = torch.cat((mem, x), dim=1)

            # auto-handle masking after appending memory tokens
            if exists(mask):
                mask = F.pad(mask, (num_mem, 0), value=True)

        x, intermediates = self.attn_layers(
            x, mask=mask, mems=mems, return_hiddens=True, **kwargs
        )
        x = self.norm(x)

        mem, x = x[:, :num_mem], x[:, num_mem:]

        out = self.to_logits(x) if not return_embeddings else x

        if return_mems:
            hiddens = intermediates.hiddens
            new_mems = (
                list(map(lambda pair: torch.cat(pair, dim=-2), zip(mems, hiddens)))
                if exists(mems)
                else hiddens
            )
            new_mems = list(
                map(lambda t: t[..., -self.max_mem_len :, :].detach(), new_mems)
            )
            return out, new_mems

        if return_attn:
            attn_maps = list(
                map(lambda t: t.post_softmax_attn, intermediates.attn_intermediates)
            )
            return out, attn_maps

        return out