Fix a lot of C++ build warnings (#16411)
Summary:
I went through my build log and did what I thought were reasonable fixes to all the C++ compilation warnings that came up
Pull Request resolved: https://github.com/pytorch/pytorch/pull/16411
Differential Revision: D13901006
Pulled By: jamesr66a
fbshipit-source-id: 02df4e3e5a5c8dd9e69ac9f065cd3f2a80645033
