Use Union type instead of Optional

In Python, Optional[List[torch.Tensor]] and Union[None, list[torch.Tensor]] are actually equivalent types. This change is to help with pyupgrade automatic changes for py39-plus.

Signed-off-by: James Butler <[email protected]>

monai/networks/nets/dynunet.py

@@ -32,7 +32,7 @@ class DynUNetSkipLayer(nn.Module):
     forward passes of the network.
     """
 
-    heads: Optional[List[torch.Tensor]]
+    heads: Union[None, List[torch.Tensor]]
 
     def __init__(self, index, downsample, upsample, next_layer, heads=None, super_head=None):
         super().__init__()