forward method

List<ValueVector> forward(List<ValueVector> x, int H, int W)

Forward pass for PatchMerging.

Takes a list of ValueVectors representing patch embeddings, assuming they form a 2D grid that can be merged. x should effectively be (H_grid * W_grid, inDim)

This pseudo-code simplifies the 2D grid reshaping for merging. In reality, you'd need the original H and W of the feature map.

Implementation

List<ValueVector> forward(List<ValueVector> x, int H, int W) {
  // H, W are the current height and width of the feature map in terms of patches.
  // e.g., if image is 224x224 and patch size is 4, initial H=56, W=56.
  // After first stage, H=28, W=28.

  assert(H % 2 == 0 && W % 2 == 0,
      "H and W must be divisible by 2 for 2x2 merging.");
  assert(x.length == H * W, "Input length must match H * W.");

  final mergedPatches = <ValueVector>[];
  for (int i = 0; i < H ~/ 2; i++) {
    for (int j = 0; j < W ~/ 2; j++) {
      // Collect 2x2 neighboring patches
      final p0 = x[i * 2 * W + j * 2];
      final p1 = x[i * 2 * W + (j * 2 + 1)];
      final p2 = x[(i * 2 + 1) * W + j * 2];
      final p3 = x[(i * 2 + 1) * W + (j * 2 + 1)];

      // Concatenate their features
      final concatenated = ValueVector(
          [...p0.values, ...p1.values, ...p2.values, ...p3.values]);

      // Normalize the concatenated features
      final normalized = norm.forward(concatenated);

      // Project to the new dimension
      final projected = projection.forward(normalized);
      mergedPatches.add(projected);
    }
  }
  return mergedPatches; // New size: (H/2 * W/2, outDim)
}