fix: small ruff issue

Author: Fabio Ferreira

monai/networks/nets/unet.py

@@ -25,6 +25,7 @@
 
 __all__ = ["UNet", "Unet"]
 
+
 class _ActivationCheckpointWrapper(nn.Module):
     def __init__(self, module: nn.Module) -> None:
         super().__init__()
@@ -35,6 +36,7 @@ def forward(self, x: torch.Tensor) -> torch.Tensor:
             return cast(torch.Tensor, checkpoint(self.module, x, use_reentrant=False))
         return cast(torch.Tensor, self.module(x))
 
+
 class UNet(nn.Module):
     """
     Enhanced version of UNet which has residual units implemented with the ResidualUnit class.