flat hash map that preserves insertion and deletion order (#25675)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/25675
This will be used to support OrderedDict in python. Modifies the existing `flat_hash_map` to preserve insertion and deletion order.
Test Plan: Imported from OSS
Differential Revision: D17440131
Pulled By: eellison
fbshipit-source-id: c7a6a290c8471627f5a061c0cca8e98ff131c9b4
