fix(aggregation): Fix raise_non_differentiable_error type (#361)

Author: ValerianRey

src/torchjd/aggregation/_utils/non_differentiable.py

@@ -6,5 +6,5 @@ def __init__(self, module: nn.Module):
         super().__init__(f"Trying to differentiate through {module}, which is not differentiable.")
 
 
-def raise_non_differentiable_error(module: nn.Module, _: tuple[Tensor, ...]) -> None:
+def raise_non_differentiable_error(module: nn.Module, _: tuple[Tensor, ...] | Tensor) -> None:
     raise NonDifferentiableError(module)