refactor affinity loss to ensure all execution branches are available

gnina/losses.py

@@ -90,7 +90,7 @@ def forward(self, input: Tensor, target: Tensor) -> Tensor:
 
  if self.reduction == "mean":
  reduced_loss = torch.mean(loss)
- elif self.reduction == "sum":
+ else: # Assertion in init ensures that reduction is "sum"
  reduced_loss = torch.sum(loss)
 
  return self.scale * reduced_loss