Use iterator of DispatchKeySet. (#44682)
Summary: Pull Request resolved: https://github.com/pytorch/pytorch/pull/44682
Test Plan: Imported from OSS
Reviewed By: ezyang
Differential Revision: D23698387
Pulled By: ailzhang
fbshipit-source-id: 4fa140db9254c2c9c342bf1c8dfd952469b0b779
