src/models/mdgt/vit_backbone.py

from __future__ import annotations

"""ViT backbone wrapper with per-layer attention map extraction.

Wraps a pretrained ViT from timm and extracts:
    - Patch tokens ``(B, P, D)`` where ``P = grid_h * grid_w``
    - Attention maps from all L layers, each ``(B, H, N, N)``
      (N = P + num_prefix_tokens; includes CLS / register tokens)

Compatible with any timm ViT variant: DINOv2, MAE, DINO, standard ViT.

DINOv2 is recommended for fingerprints — attention maps overlap IoU ~0.41
with minutiae locations (DINOv2-FP), confirming that the network discovers
salient keypoints similar to classical minutiae extraction.
"""

import torch
import torch.nn as nn

try:
    import timm
except ImportError:
    timm = None


class ViTBackbone(nn.Module):
    """Pretrained ViT backbone with per-layer attention extraction.

    At forward time, runs the ViT block-by-block and captures pre-dropout
    attention weights from each layer.  When ``freeze=True``, the entire
    forward runs under ``torch.no_grad()`` for memory efficiency.

    Parameters
    ----------
    model_name : str
        timm model identifier (e.g. ``"vit_base_patch14_dinov2.lvd142m"``).
    pretrained : bool
        Load pretrained weights from timm hub.
    freeze : bool
        Freeze all ViT parameters (feature-extraction mode).
    image_size : int
        Input resolution (square).  Position embeddings are interpolated
        automatically if this differs from the model's training resolution.
    """

    def __init__(
        self,
        model_name: str = "vit_base_patch14_dinov2.lvd142m",
        pretrained: bool = True,
        freeze: bool = True,
        image_size: int = 224,
    ):
        super().__init__()
        if timm is None:
            raise ImportError(
                "timm is required for ViT backbone: pip install timm"
            )

        self.vit = timm.create_model(
            model_name,
            pretrained=pretrained,
            img_size=image_size,
            num_classes=0,
        )

        self._frozen = freeze
        if freeze:
            for p in self.vit.parameters():
                p.requires_grad = False

        self.embed_dim: int = self.vit.embed_dim
        self.grid_size: tuple[int, int] = self.vit.patch_embed.grid_size
        self.num_patches: int = self.grid_size[0] * self.grid_size[1]
        self.num_prefix_tokens: int = getattr(
            self.vit, "num_prefix_tokens", 1
        )

    # ------------------------------------------------------------------
    def forward(
        self, images: torch.Tensor
    ) -> tuple[torch.Tensor, list[torch.Tensor]]:
        """
        Args:
            images: ``(B, 3, H, W)`` RGB input.
                    Grayscale images should be repeated to 3 channels
                    *before* calling this method.

        Returns:
            patch_tokens: ``(B, P, D)`` — patch features only (no CLS /
                          register tokens).
            attn_maps:    list of *L* tensors, each ``(B, H, N, N)`` with
                          pre-dropout attention weights.
        """
        if self._frozen:
            with torch.no_grad():
                return self._forward_impl(images)
        return self._forward_impl(images)

    # ------------------------------------------------------------------
    def _forward_impl(
        self, images: torch.Tensor
    ) -> tuple[torch.Tensor, list[torch.Tensor]]:
        # 1. Patch embedding
        x = self.vit.patch_embed(images)

        # 2. CLS / register tokens + positional embedding
        #    timm >= 0.9 exposes _pos_embed(); fall back for older versions.
        if hasattr(self.vit, "_pos_embed"):
            x = self.vit._pos_embed(x)
        else:
            cls = self.vit.cls_token.expand(x.shape[0], -1, -1)
            x = torch.cat([cls, x], dim=1)
            x = self.vit.pos_drop(x + self.vit.pos_embed)

        # 3. Transformer blocks — extract attention at every layer
        attn_maps: list[torch.Tensor] = []
        for blk in self.vit.blocks:
            x, attn = self._block_with_attn(blk, x)
            attn_maps.append(attn)

        # 4. Final layer norm
        x = self.vit.norm(x)

        # 5. Strip prefix tokens (CLS + optional registers)
        patch_tokens = x[:, self.num_prefix_tokens :]  # (B, P, D)
        return patch_tokens, attn_maps

    # ------------------------------------------------------------------
    @staticmethod
    def _block_with_attn(
        blk: nn.Module, x: torch.Tensor
    ) -> tuple[torch.Tensor, torch.Tensor]:
        """Forward one ViT block; return (updated x, attention weights).

        Re-implements the block forward to capture pre-dropout attention.
        Handles both old-style (shared drop_path) and new-style timm
        blocks (drop_path1/2, ls1/2, q_norm/k_norm).
        """
        attn_mod = blk.attn
        B, N, C = x.shape
        num_heads = attn_mod.num_heads
        scale = attn_mod.scale

        # ---- Self-attention with weight extraction ----
        residual = x
        x_norm = blk.norm1(x)

        qkv = attn_mod.qkv(x_norm)
        inner_dim = qkv.shape[-1] // 3
        head_dim = inner_dim // num_heads
        qkv = qkv.reshape(B, N, 3, num_heads, head_dim).permute(
            2, 0, 3, 1, 4
        )
        q, k, v = qkv.unbind(0)

        # QK normalization (DINOv2 / timm >= 0.9)
        if hasattr(attn_mod, "q_norm") and attn_mod.q_norm is not None:
            q = attn_mod.q_norm(q)
        if hasattr(attn_mod, "k_norm") and attn_mod.k_norm is not None:
            k = attn_mod.k_norm(k)

        attn_weights = (q @ k.transpose(-2, -1)) * scale
        attn_weights = attn_weights.softmax(dim=-1)  # (B, H, N, N)

        # Keep pre-dropout copy for TRAM (detached — no grad needed)
        attn_out = attn_weights.detach()

        y = (attn_mod.attn_drop(attn_weights) @ v)
        y = y.transpose(1, 2).reshape(B, N, C)
        y = attn_mod.proj(y)
        y = attn_mod.proj_drop(y)

        # Layer scale + drop path (version-agnostic)
        if hasattr(blk, "ls1"):
            y = blk.ls1(y)
        dp1 = getattr(blk, "drop_path1", getattr(blk, "drop_path", None))
        x = residual + (dp1(y) if dp1 is not None else y)

        # ---- FFN ----
        residual = x
        ffn_out = blk.mlp(blk.norm2(x))
        if hasattr(blk, "ls2"):
            ffn_out = blk.ls2(ffn_out)
        dp2 = getattr(blk, "drop_path2", getattr(blk, "drop_path", None))
        x = residual + (dp2(ffn_out) if dp2 is not None else ffn_out)

        return x, attn_out