add whitelisted clang-format checks (#15254)

add whitelisted clang-format checks (#15254)

Summary:
This PR adds clang-format automation:
- It only checks on whitelisted files, so we can enable incrementally without noise
- There is a pre-commit hook provided that will do the same check, plus prompt users to apply the clang-format changes (no change is made without the user agreeing).

My plan is to migrate over whole files at a time, clang-formatting them and then adding them to the whitelist. Doing it this way should avoid too many merge pains (the most you'll have to is run clang-format on the affected file before rebasing).
Pull Request resolved: https://github.com/pytorch/pytorch/pull/15254

Differential Revision: D13515888

Pulled By: suo

fbshipit-source-id: d098eabcc97aa228c4dfce8fc096c3b5a45b591f