[dtensor] handle the case where output of op is Optional[Tensor] (#90241)
Observed by @aazzolini, some op might have Optional[Tensor] returns
where it return None (i.e. native_layer_norm_backward), it's a mismatch
between C++ aten op signature and python None, but we need to handle it
in the python side
Pull Request resolved: https://github.com/pytorch/pytorch/pull/90241
Approved by: https://github.com/aazzolini
