Keep Reducer hooks in a vector instead of an unordered_map (#21783)
Summary:
kuttas pointed out that the DDP Reducer only needs to remember `uintptr, Function` pairs, and hence does not need a nunordered map as added by https://github.com/pytorch/pytorch/issues/21591. Using a vector should speed it up a bit.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/21783
Differential Revision: D15854312
Pulled By: mrshenli
fbshipit-source-id: 153ba035b8d658c7878a613f16a42de977d89c43
