fix aten::grad to return optional list (#29577)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/29577
`torch.autograd.grad` can return none is one of the input is not in the
autograd graph or not requires_grad, this fix it so that it return a
list of optional tensor instead of list of tensor.
This might have BC issue unfortunately, but I think it's rare both
internal and external (only training use it, and most of the training
use backward, instead of autograd.grad), so whitelist it.
Test Plan: Imported from OSS
Differential Revision: D18491642
fbshipit-source-id: d32b2b3446cf9e8b9a98f6d203a21a75643d8991
