transformer block local window attention

Signed-off-by: vgrau98 <[email protected]>

Author: vgrau98

monai/networks/blocks/transformerblock.py

@@ -11,7 +11,11 @@
 
 from __future__ import annotations
 
+from typing import Tuple
+
+import torch
 import torch.nn as nn
+import torch.nn.functional as F
 
 from monai.networks.blocks.mlp import MLPBlock
 from monai.networks.blocks.selfattention import SABlock
@@ -31,6 +35,7 @@ def __init__(
         dropout_rate: float = 0.0,
         qkv_bias: bool = False,
         save_attn: bool = False,
+        window_size: int = 0,
     ) -> None:
         """
         Args:
@@ -40,6 +45,10 @@ def __init__(
             dropout_rate (float, optional): fraction of the input units to drop. Defaults to 0.0.
             qkv_bias (bool, optional): apply bias term for the qkv linear layer. Defaults to False.
             save_attn (bool, optional): to make accessible the attention matrix. Defaults to False.
+            window_size (int): Window size for local attention as used in Segment Anything https://arxiv.org/abs/2304.02643.
+                If 0, global attention used. Only 2D inputs are supported for local attention (window_size > 0).
+                If local attention is used, the input tensor should have the following shape during the forward pass: [B, H, W, C].
+                See https://github.com/facebookresearch/segment-anything/blob/main/segment_anything/modeling/image_encoder.py.
 
         """
 
@@ -55,8 +64,70 @@ def __init__(
         self.norm1 = nn.LayerNorm(hidden_size)
         self.attn = SABlock(hidden_size, num_heads, dropout_rate, qkv_bias, save_attn)
         self.norm2 = nn.LayerNorm(hidden_size)
+        self.window_size = window_size
 
     def forward(self, x):
-        x = x + self.attn(self.norm1(x))
+        shortcut = x
+        x = self.norm1(x)
+        # Window partition
+        if self.window_size > 0:
+            H, W = x.shape[1], x.shape[2]
+            x, pad_hw = window_partition(x, self.window_size)
+
+        x = self.attn(x)
+        # Reverse window partition
+        if self.window_size > 0:
+            x = window_unpartition(x, self.window_size, pad_hw, (H, W))
+
+        x = shortcut + x
         x = x + self.mlp(self.norm2(x))
         return x
+
+
+def window_partition(x: torch.Tensor, window_size: int) -> Tuple[torch.Tensor, Tuple[int, int]]:
+    """
+    Partition into non-overlapping windows with padding if needed. Support only 2D.
+    Args:
+        x (tensor): input tokens with [B, H, W, C].
+        window_size (int): window size.
+
+    Returns:
+        windows: windows after partition with [B * num_windows, window_size, window_size, C].
+        (Hp, Wp): padded height and width before partition
+    """
+    B, H, W, C = x.shape
+
+    pad_h = (window_size - H % window_size) % window_size
+    pad_w = (window_size - W % window_size) % window_size
+    if pad_h > 0 or pad_w > 0:
+        x = F.pad(x, (0, 0, 0, pad_w, 0, pad_h))
+    Hp, Wp = H + pad_h, W + pad_w
+
+    x = x.view(B, Hp // window_size, window_size, Wp // window_size, window_size, C)
+    windows = x.permute(0, 1, 3, 2, 4, 5).contiguous().view(-1, window_size, window_size, C)
+    return windows, (Hp, Wp)
+
+
+def window_unpartition(
+    windows: torch.Tensor, window_size: int, pad_hw: Tuple[int, int], hw: Tuple[int, int]
+) -> torch.Tensor:
+    """
+    Window unpartition into original sequences and removing padding.
+    Args:
+        windows (tensor): input tokens with [B * num_windows, window_size, window_size, C].
+        window_size (int): window size.
+        pad_hw (Tuple): padded height and width (Hp, Wp).
+        hw (Tuple): original height and width (H, W) before padding.
+
+    Returns:
+        x: unpartitioned sequences with [B, H, W, C].
+    """
+    Hp, Wp = pad_hw
+    H, W = hw
+    B = windows.shape[0] // (Hp * Wp // window_size // window_size)
+    x = windows.view(B, Hp // window_size, Wp // window_size, window_size, window_size, -1)
+    x = x.permute(0, 1, 3, 2, 4, 5).contiguous().view(B, Hp, Wp, -1)
+
+    if Hp > H or Wp > W:
+        x = x[:, :H, :W, :].contiguous()
+    return x