Fix PEP448 for python < 3.8 (#527)

Co-authored-by: danthe3rd <danthe3rd@users.noreply.github.com>

xformers/ops/unbind.py

@@ -96,7 +96,7 @@ def forward(ctx, dim: int, *tensors: torch.Tensor):
     @classmethod
     # type: ignore
     def backward(cls, ctx, grad: torch.Tensor):
-        return None, *grad.unbind(dim=ctx.dim)
+        return None, *(grad.unbind(dim=ctx.dim))
 
 
 def unbind(x: torch.Tensor, dim: int) -> Tuple[torch.Tensor, ...]: