Update flat_hash_map (#15367)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/15367
This updates flat_hash_map and fixes an issue with singletons across library boundaries
(see the PRs linked at the top of the file)
Reviewed By: ezyang
Differential Revision: D13510912
fbshipit-source-id: e90a297a7a2d69ae3fe48e4fcd8a44ad4b81292a
