Fix some TPU (XLA) issues with swin transformer v2

timm/models/swin_transformer_v2_cr.py

@@ -392,13 +392,15 @@ class SwinTransformerBlock(nn.Module):
         x = x.view(B, H, W, C)
 
         # cyclic shift
+        sh, sw = self.shift_size
         if any(self.shift_size):
-            shifted_x = torch.roll(x, shifts=(-self.shift_size[0], -self.shift_size[1]), dims=(1, 2))
-        else:
-            shifted_x = x
+            # FIXME PyTorch XLA needs cat impl, roll not lowered
+            # x = torch.cat([x[:, sh:], x[:, :sh]], dim=1)
+            # x = torch.cat([x[:, :, sw:], x[:, :, :sw]], dim=2)
+            x = torch.roll(x, shifts=(-sh, -sw), dims=(1, 2))
 
         # partition windows
-        x_windows = window_partition(shifted_x, self.window_size)  # num_windows * B, window_size, window_size, C
+        x_windows = window_partition(x, self.window_size)  # num_windows * B, window_size, window_size, C
         x_windows = x_windows.view(-1, self.window_size[0] * self.window_size[1], C)
 
         # W-MSA/SW-MSA
@@ -406,13 +408,14 @@ class SwinTransformerBlock(nn.Module):
 
         # merge windows
         attn_windows = attn_windows.view(-1, self.window_size[0], self.window_size[1], C)
-        shifted_x = window_reverse(attn_windows, self.window_size, self.feat_size)  # B H' W' C
+        x = window_reverse(attn_windows, self.window_size, self.feat_size)  # B H' W' C
 
         # reverse cyclic shift
         if any(self.shift_size):
-            x = torch.roll(shifted_x, shifts=self.shift_size, dims=(1, 2))
-        else:
-            x = shifted_x
+            # FIXME PyTorch XLA needs cat impl, roll not lowered
+            # x = torch.cat([x[:, -sh:], x[:, :-sh]], dim=1)
+            # x = torch.cat([x[:, :, -sw:], x[:, :, :-sw]], dim=2)
+            x = torch.roll(x, shifts=(sh, sw), dims=(1, 2))
 
         x = x.view(B, L, C)
         return x
@@ -452,8 +455,10 @@ class PatchMerging(nn.Module):
         Returns:
             output (torch.Tensor): Output tensor of the shape [B, 2 * C, H // 2, W // 2]
         """
-        x = bchw_to_bhwc(x).unfold(dimension=1, size=2, step=2).unfold(dimension=2, size=2, step=2)
-        x = x.permute(0, 1, 2, 5, 4, 3).flatten(3)  # permute maintains compat with ch order in official swin impl
+        B, C, H, W = x.shape
+        # unfold + BCHW -> BHWC together
+        # ordering, 5, 3, 1 instead of 3, 5, 1 maintains compat with original swin v1 merge
+        x = x.reshape(B, C, H // 2, 2, W // 2, 2).permute(0, 2, 4, 5, 3, 1).flatten(3)
         x = self.norm(x)
         x = bhwc_to_bchw(self.reduction(x))
         return x