Fix `_upsample_2d` (#535)

* Fix _upsample_2d
Co-authored-by: ydshieh <ydshieh@users.noreply.github.com>

src/diffusers/models/resnet.py

@@ -149,7 +149,6 @@ class FirUpsample2D(nn.Module):
 
             stride = (factor, factor)
             # Determine data dimensions.
-            stride = [1, 1, factor, factor]
             output_shape = ((x.shape[2] - 1) * factor + convH, (x.shape[3] - 1) * factor + convW)
             output_padding = (
                 output_shape[0] - (x.shape[2] - 1) * stride[0] - convH,
@@ -161,7 +160,7 @@ class FirUpsample2D(nn.Module):
 
             # Transpose weights.
             weight = torch.reshape(weight, (num_groups, -1, inC, convH, convW))
-            weight = weight[..., ::-1, ::-1].permute(0, 2, 1, 3, 4)
+            weight = torch.flip(weight, dims=[3, 4]).permute(0, 2, 1, 3, 4)
             weight = torch.reshape(weight, (num_groups * inC, -1, convH, convW))
 
             x = F.conv_transpose2d(x, weight, stride=stride, output_padding=output_padding, padding=0)