Give more accurate error message (#1358)
Summary:
Pull Request resolved: https://github.com/pytorch/benchmark/pull/1358
Show more accurate message to distinguish grad does not exist or grad mismatch.
Reviewed By: jianyuh, rkindi, jiawenliu64
Differential Revision: D42361170
fbshipit-source-id: e5950bc0c9bbb53f6f8b7b625dba842ac36f8c3c
