models/optical_compressor.py

"""
OpticalCompressor: learned Conv2D compression for Qwen3-VL visual tokens.

Inserted AFTER model.visual() (ViT + Merger), BEFORE the LLM.
Reduces ~6700 merged visual tokens → target_tokens (e.g. 256).

The key idea from DeepSeek-OCR: compress at the encoding stage via
learned spatial downsampling, rather than pruning at inference time.
"""

import math
import torch
import torch.nn as nn


class OpticalCompressor(nn.Module):
    def __init__(self, hidden_dim=3584, target_tokens=256, num_refine_layers=1):
        """
        Args:
            hidden_dim: LLM hidden dimension (3584 for Qwen3-VL-8B after merger)
            target_tokens: number of output tokens (default 256 = 16x16 grid)
            num_refine_layers: number of Transformer refinement layers
        """
        super().__init__()
        self.hidden_dim = hidden_dim
        self.target_tokens = target_tokens
        self.target_h = int(math.sqrt(target_tokens))
        self.target_w = self.target_h
        assert self.target_h * self.target_w == target_tokens

        # Stage 1: Spatial Conv compression
        # Input: [B, D, H, W] → progressive downsampling
        # Using depthwise-separable conv to keep params manageable
        # Note: GroupNorm(1, D) == LayerNorm over channels, works on [B,C,H,W]
        num_groups = 32 if hidden_dim % 32 == 0 else 1
        self.compress = nn.Sequential(
            # Block 1: reduce spatial by 2x
            nn.Conv2d(hidden_dim, hidden_dim, kernel_size=3, stride=2,
                      padding=1, groups=hidden_dim, bias=False),
            nn.Conv2d(hidden_dim, hidden_dim, kernel_size=1, bias=False),
            nn.GroupNorm(num_groups, hidden_dim),
            nn.GELU(),
            # Block 2: reduce spatial by 2x again
            nn.Conv2d(hidden_dim, hidden_dim, kernel_size=3, stride=2,
                      padding=1, groups=hidden_dim, bias=False),
            nn.Conv2d(hidden_dim, hidden_dim, kernel_size=1, bias=False),
            nn.GroupNorm(num_groups, hidden_dim),
            nn.GELU(),
        )

        # Stage 2: Adaptive pool to fixed grid
        self.pool = nn.AdaptiveAvgPool2d((self.target_h, self.target_w))

        # Stage 3: Transformer refinement (recover inter-token relationships)
        self.refine = nn.ModuleList([
            nn.TransformerEncoderLayer(
                d_model=hidden_dim, nhead=8,
                dim_feedforward=hidden_dim * 2,
                dropout=0.1, activation="gelu",
                batch_first=True, norm_first=True,
            )
            for _ in range(num_refine_layers)
        ])

        # Learnable position embedding for output tokens
        self.pos_embed = nn.Parameter(
            torch.randn(1, target_tokens, hidden_dim) * 0.02
        )

        self._init_weights()

    def _init_weights(self):
        for m in self.modules():
            if isinstance(m, nn.Conv2d):
                nn.init.kaiming_normal_(m.weight, mode="fan_out")
                if m.bias is not None:
                    nn.init.zeros_(m.bias)
            elif isinstance(m, nn.Linear):
                nn.init.trunc_normal_(m.weight, std=0.02)
                if m.bias is not None:
                    nn.init.zeros_(m.bias)

    def forward(self, visual_embeds, grid_thw):
        """
        Args:
            visual_embeds: [total_tokens, hidden_dim] — flat output from model.visual()
                           Contains tokens for ALL images in the batch concatenated.
            grid_thw: [num_images, 3] — (temporal, height, width) for each image
                      after the merger's spatial merge.

        Returns:
            compressed: [total_compressed, hidden_dim] — flat compressed tokens
            new_grid_thw: [num_images, 3] — new grid dimensions (1, target_h, target_w)
        """
        device = visual_embeds.device
        compressed_list = []
        offset = 0

        for i in range(grid_thw.shape[0]):
            t, h, w = grid_thw[i].tolist()
            t, h, w = int(t), int(h), int(w)
            n_tokens = t * h * w
            img_tokens = visual_embeds[offset:offset + n_tokens]  # [n, D]
            offset += n_tokens

            # Reshape to spatial: take first temporal frame for 2D conv
            # For images (not video), t=1 always
            x = img_tokens.reshape(t * h, w, self.hidden_dim)
            # Merge temporal into height for simplicity
            x = x.reshape(1, t * h, w, self.hidden_dim)  # [1, H, W, D]
            x = x.permute(0, 3, 1, 2)  # [1, D, H, W]

            # Stage 1: Conv compression
            x = self.compress(x)  # [1, D, H', W']

            # Stage 2: Pool to fixed size
            x = self.pool(x)  # [1, D, target_h, target_w]

            x = x.flatten(2).transpose(1, 2)  # [1, target_tokens, D]

            # Stage 3: Add position embedding + refine
            x = x + self.pos_embed.to(x.dtype)
            for layer in self.refine:
                x = layer(x)

            compressed_list.append(x.squeeze(0))  # [target_tokens, D]

        compressed = torch.cat(compressed_list, dim=0)  # [total_compressed, D]
        new_grid_thw = torch.tensor(
            [[1, self.target_h, self.target_w]] * grid_thw.shape[0],
            device=device, dtype=grid_thw.dtype,
        )
        return compressed, new_grid_thw

    def count_parameters(self):
        total = sum(p.numel() for p in self.parameters())
        trainable = sum(p.numel() for p in self.parameters() if p.requires_grad)
        return {"total": total, "trainable": trainable}