Allow empty Variables to be saved for backwards (#23618)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/23618
For example: `save_for_backward({Variable(), x, Variable()})` should be allowed, so that this is consistent with the python API behaviour.
Test Plan: Added a test similar to the python test `test_save_none_for_backward` from test_autograd.py.
Differential Revision: D16589402
fbshipit-source-id: 847544ad8fc10772954d8629ad5a62bfdc1a66c1
