model_core.py

import torch
import torch.nn as nn
import torch.nn.functional as F
import numpy as np
from timm.models.layers import to_2tuple

class PatchEmbed_new(nn.Module):
    """ Flexible Image to Patch Embedding
    """
    def __init__(self, img_size=224, patch_size=16, in_chans=3, embed_dim=768, stride=16):
        super().__init__()
        img_size = to_2tuple(img_size)
        patch_size = to_2tuple(patch_size)
        stride = to_2tuple(stride)
        
        self.img_size = img_size
        self.patch_size = patch_size

        self.proj = nn.Conv2d(in_chans, embed_dim, kernel_size=patch_size, stride=stride) # with overlapped patches

    def forward(self, x):
        x = self.proj(x)
        x = x.flatten(2).transpose(1, 2)
        return x


def get_2d_sincos_pos_embed_flexible(embed_dim, grid_size, cls_token=False):
    """
    grid_size: int of the grid height and width
    return:
    pos_embed: [grid_size*grid_size, embed_dim] or [1+grid_size*grid_size, embed_dim] (w/ or w/o cls_token)
    """
    grid_h = np.arange(grid_size[0], dtype=np.float32)
    grid_w = np.arange(grid_size[1], dtype=np.float32)
    grid = np.meshgrid(grid_w, grid_h)  # here w goes first
    grid = np.stack(grid, axis=0)

    grid = grid.reshape([2, 1, grid_size[0], grid_size[1]])
    pos_embed = get_2d_sincos_pos_embed_from_grid(embed_dim, grid)
    if cls_token:
        pos_embed = np.concatenate([np.zeros([1, embed_dim]), pos_embed], axis=0)
    return pos_embed


def get_2d_sincos_pos_embed_from_grid(embed_dim, grid):
    assert embed_dim % 2 == 0

    # use half of dimensions to encode grid_h
    emb_h = get_1d_sincos_pos_embed_from_grid(embed_dim // 2, grid[0])  # (H*W, D/2)
    emb_w = get_1d_sincos_pos_embed_from_grid(embed_dim // 2, grid[1])  # (H*W, D/2)

    emb = np.concatenate([emb_h, emb_w], axis=1)  # (H*W, D)
    return emb


def get_1d_sincos_pos_embed_from_grid(embed_dim, pos):
    """
    embed_dim: output dimension for each position
    pos: a list of positions to be encoded: size (M,)
    out: (M, D)
    """
    assert embed_dim % 2 == 0
    omega = np.arange(embed_dim // 2, dtype=np.float32)
    omega /= embed_dim / 2.0
    omega = 1.0 / 10000 ** omega  # (D/2,)

    pos = pos.reshape(-1)  # (M,)
    out = np.einsum("m,d->md", pos, omega)  # (M, D/2), outer product

    emb_sin = np.sin(out)  # (M, D/2)
    emb_cos = np.cos(out)  # (M, D/2)

    emb = np.concatenate([emb_sin, emb_cos], axis=1)  # (M, D)
    return emb


class FixedPositionalEncoder(nn.Module):
    def __init__(self, pos_embed):
        super().__init__()
        self.positions = pos_embed

    def forward(self, x, padding_mask): 
        return self.positions


class AltBlock(nn.Module):
    def __init__(
        self,
        dim,
        num_heads,
        mlp_ratio=4.0,
        qkv_bias=False,
        qk_scale=None,
        drop=0.0,
        attn_drop=0.0,
        mlp_drop=0.0,
        post_mlp_drop=0.0,
        drop_path=0.0,
        act_layer=nn.GELU,
        norm_layer=nn.LayerNorm,
        layer_norm_first=True,
        ffn_targets=False,
        cosine_attention=False,
    ):
        super().__init__()

        self.layer_norm_first = layer_norm_first
        self.ffn_targets = ffn_targets

        from timm.models.vision_transformer import DropPath, Mlp

        self.norm1 = norm_layer(dim)
        self.attn = AltAttention(
            dim,
            num_heads=num_heads,
            qkv_bias=qkv_bias,
            qk_scale=qk_scale,
            attn_drop=attn_drop,
            proj_drop=drop,
            cosine_attention=cosine_attention,
        )

        self.drop_path = DropPath(drop_path) if drop_path > 0.0 else nn.Identity()
        self.norm2 = norm_layer(dim)
        mlp_hidden_dim = int(dim * mlp_ratio)
        self.mlp = Mlp(
            in_features=dim,
            hidden_features=mlp_hidden_dim,
            act_layer=act_layer,
            drop=mlp_drop,
        )
        self.post_mlp_dropout = nn.Dropout(post_mlp_drop, inplace=False)

    def forward(self, x, padding_mask=None, alibi_bias=None):
        if self.layer_norm_first:
            x = x + self.drop_path(self.attn(self.norm1(x), padding_mask, alibi_bias))
            r = x = self.mlp(self.norm2(x))
            t = x
            x = r + self.drop_path(self.post_mlp_dropout(x))
            if not self.ffn_targets:
                t = x
        else:
            x = x + self.drop_path(self.attn(x, padding_mask, alibi_bias))
            r = x = self.norm1(x)
            x = self.mlp(x)
            t = x
            x = self.norm2(r + self.drop_path(self.post_mlp_dropout(x)))
            if not self.ffn_targets:
                t = x

        return x, t


class AltAttention(nn.Module):
    def __init__(
        self,
        dim,
        num_heads=8,
        qkv_bias=False,
        qk_scale=None,
        attn_drop=0.0,
        proj_drop=0.0,
        cosine_attention=False,
    ):
        super().__init__()
        self.num_heads = num_heads
        head_dim = dim // num_heads
        self.scale = qk_scale or head_dim ** -0.5

        self.qkv = nn.Linear(dim, dim * 3, bias=qkv_bias)
        self.attn_drop = nn.Dropout(attn_drop)
        self.proj = nn.Linear(dim, dim)
        self.proj_drop = nn.Dropout(proj_drop)

        self.cosine_attention = cosine_attention

        if cosine_attention:
            self.logit_scale = nn.Parameter(
                torch.log(10 * torch.ones((num_heads, 1, 1))), requires_grad=True
            )

    def forward(self, x, padding_mask=None, alibi_bias=None):
        B, N, C = x.shape
        qkv = (
            self.qkv(x)
            .reshape(B, N, 3, self.num_heads, C // self.num_heads)
            .permute(2, 0, 3, 1, 4)  # qkv x B x H x L x D
        )
        q, k, v = (
            qkv[0],
            qkv[1],
            qkv[2],
        )  # make torchscript happy (cannot use tensor as tuple)

        dtype = q.dtype

        if self.cosine_attention:
            # cosine attention
            attn = F.normalize(q, dim=-1) @ F.normalize(k, dim=-1).transpose(-2, -1)
            logit_scale = torch.clamp(
                self.logit_scale, max=torch.log(torch.tensor(1.0 / 0.01))
            ).exp()
            attn = attn * logit_scale
        else:
            q = q * self.scale
            attn = q @ k.transpose(-2, -1)

        if alibi_bias is not None:
            attn = attn.type_as(alibi_bias)
            attn[:, : alibi_bias.size(1)] += alibi_bias

        if padding_mask is not None and padding_mask.any():
            attn = attn.masked_fill(
                padding_mask.unsqueeze(1).unsqueeze(2).to(torch.bool),
                float("-inf"),
            )

        attn = attn.softmax(dim=-1, dtype=torch.float32).to(dtype=dtype)
        attn = self.attn_drop(attn)
        x = (attn @ v).transpose(1, 2)  #
        x = x.reshape(B, N, C)
        x = self.proj(x)
        x = self.proj_drop(x)
        return x