fix: resolve mypy union-attr error in unified_focal_loss

Signed-off-by: Rusheel Sharma <rusheelhere@gmail.com>

Author: Rusheel86

monai/losses/unified_focal_loss.py

@@ -170,15 +170,17 @@ def forward(self, y_pred: torch.Tensor, y_true: torch.Tensor) -> torch.Tensor:
         cross_entropy = -y_true * torch.log(y_pred)
 
         # Build mask from original labels if available
-        spatial_mask = None
+        spatial_mask: torch.Tensor | None = None
         if self.ignore_index is not None:
             if original_y_true is not None and self.to_onehot_y:
                 spatial_mask = (original_y_true != self.ignore_index).float()
             elif self.ignore_index < y_true.shape[1]:
                 spatial_mask = 1.0 - y_true[:, self.ignore_index : self.ignore_index + 1]
             else:
                 spatial_mask = (y_true.sum(dim=1, keepdim=True) > 0).float()
-            cross_entropy = cross_entropy * spatial_mask.expand_as(cross_entropy)
+
+            if spatial_mask is not None:
+                cross_entropy = cross_entropy * spatial_mask.expand_as(cross_entropy)
 
         back_ce = torch.pow(1 - y_pred[:, 0], self.gamma) * cross_entropy[:, 0]
         back_ce = (1 - self.delta) * back_ce