Precommit hook: just warn if no clang-tidy (#15514)
Summary:
The precommit hook shouldn't hard fail if there's no `clang-tidy`, just warn and omit the check.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/15514
Differential Revision: D13545776
Pulled By: suo
fbshipit-source-id: 9bf3f8ee18703c6d1a39eb7776092fb5e120d2a1
