[forward ad] Fix codegen to ignore undefined outputs (#81114)
I don't think there's a way to avoid functions returning undefined tensors as outputs, so codegen will have to detect them before calling _set_fw_grad. Alternatively, we can just make calling _set_fw_grad with undefined self a no-op, but I'm biasing toward keeping _set_fw_grad more strict in case it is called in other areas.
Fixes https://github.com/pytorch/pytorch/issues/81111
Pull Request resolved: https://github.com/pytorch/pytorch/pull/81114
Approved by: https://github.com/albanD
