ldm/modules/encoders/xf.py

"""
Transformer implementation adapted from CLIP ViT:
https://github.com/openai/CLIP/blob/4c0275784d6d9da97ca1f47eaaee31de1867da91/clip/model.py
"""

import math

import torch as th
import torch.nn as nn


def convert_module_to_f16(l):
    """
    Convert primitive modules to float16.
    """
    if isinstance(l, (nn.Linear, nn.Conv2d, nn.ConvTranspose2d)):
        l.weight.data = l.weight.data.half()
        if l.bias is not None:
            l.bias.data = l.bias.data.half()


class LayerNorm(nn.LayerNorm):
    """
    Implementation that supports fp16 inputs but fp32 gains/biases.
    """

    def forward(self, x: th.Tensor):
        return super().forward(x.float()).to(x.dtype)


class MultiheadAttention(nn.Module):
    def __init__(self, n_ctx, width, heads):
        super().__init__()
        self.n_ctx = n_ctx
        self.width = width
        self.heads = heads
        self.c_qkv = nn.Linear(width, width * 3)
        self.c_proj = nn.Linear(width, width)
        self.attention = QKVMultiheadAttention(heads, n_ctx)

    def forward(self, x):
        x = self.c_qkv(x)
        x = self.attention(x)
        x = self.c_proj(x)
        return x


class MLP(nn.Module):
    def __init__(self, width):
        super().__init__()
        self.width = width
        self.c_fc = nn.Linear(width, width * 4)
        self.c_proj = nn.Linear(width * 4, width)
        self.gelu = nn.GELU()

    def forward(self, x):
        return self.c_proj(self.gelu(self.c_fc(x)))


class QKVMultiheadAttention(nn.Module):
    def __init__(self, n_heads: int, n_ctx: int):
        super().__init__()
        self.n_heads = n_heads
        self.n_ctx = n_ctx

    def forward(self, qkv):
        bs, n_ctx, width = qkv.shape
        attn_ch = width // self.n_heads // 3
        scale = 1 / math.sqrt(math.sqrt(attn_ch))
        qkv = qkv.view(bs, n_ctx, self.n_heads, -1)
        q, k, v = th.split(qkv, attn_ch, dim=-1)
        weight = th.einsum(
            "bthc,bshc->bhts", q * scale, k * scale
        )  # More stable with f16 than dividing afterwards
        wdtype = weight.dtype
        weight = th.softmax(weight.float(), dim=-1).type(wdtype)
        return th.einsum("bhts,bshc->bthc", weight, v).reshape(bs, n_ctx, -1)


class ResidualAttentionBlock(nn.Module):
    def __init__(
        self,
        n_ctx: int,
        width: int,
        heads: int,
    ):
        super().__init__()

        self.attn = MultiheadAttention(
            n_ctx,
            width,
            heads,
        )
        self.ln_1 = LayerNorm(width)
        self.mlp = MLP(width)
        self.ln_2 = LayerNorm(width)

    def forward(self, x: th.Tensor):
        x = x + self.attn(self.ln_1(x))
        x = x + self.mlp(self.ln_2(x))
        return x


class Transformer(nn.Module):
    def __init__(
        self,
        n_ctx: int,
        width: int,
        layers: int,
        heads: int,
    ):
        super().__init__()
        self.n_ctx = n_ctx
        self.width = width
        self.layers = layers
        self.resblocks = nn.ModuleList(
            [
                ResidualAttentionBlock(
                    n_ctx,
                    width,
                    heads,
                )
                for _ in range(layers)
            ]
        )

    def forward(self, x: th.Tensor):
        for block in self.resblocks:
            x = block(x)
        return x