Torchhub: rewrite commit hash check to avoid using unnecessary GitHub API credits...

Torchhub: rewrite commit hash check to avoid using unnecessary GitHub API credits (#64362)

Summary:
This PR adds more detailed error messages to torchhub if the commit hash validation goes wrong, providing suggestions to the users on how to resolve the issue.

It also documents why such validation is important.

EDIT: it also avoids validatating some stuff when we know "stuff" isn't a commit since there's no risk in this case

CC malfet mthrok

cc nairbv NicolasHug

Pull Request resolved: https://github.com/pytorch/pytorch/pull/64362

Reviewed By: gchanan, malfet

Differential Revision: D30731191

Pulled By: NicolasHug

fbshipit-source-id: d1ee7c2ef2591dd7a5291977af1635ada2552d1b