require non-empty inputs to grad() calls in the API (#52016)
Summary:
The grad() function needs to return the updated values, and hence
needs a non-empty inputs to populate.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/52016
Test Plan:
Passes Python and C++ unit tests, and added new tests to catch this behavior.
Fixes https://github.com/pytorch/pytorch/issues/47061
Reviewed By: albanD
Differential Revision: D26406444
Pulled By: dagitses
fbshipit-source-id: 023aeca9a40cd765c5bad6a1a2f8767a33b75a1a
