Fix unreachable validation for gradcheck (#37915)
Summary:
Hi, I found the validation that is unreachable in `gradcheck` function :)
Pull Request resolved: https://github.com/pytorch/pytorch/pull/37915
Differential Revision: D21551661
Pulled By: albanD
fbshipit-source-id: 8acadcc09cd2afb539061eda0ca5e98860e321eb
