Fix verbose compiler warning in flat_hash_map (#17562)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/17562
fixes https://github.com/pytorch/pytorch/issues/17332
Reviewed By: ezyang
Differential Revision: D14254499
fbshipit-source-id: 9d5d7408c2ce510ac20cd438c6514dc2bbe3a854
