free up dispatch key space (in C++) (#72827)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/72827
Reland of D34034848 (https://github.com/pytorch/pytorch/commit/66902560216740db6f38cb7a8458bd29c213f296)
ghstack-source-id: 152161452
Test Plan: Confirm that Milan tests are passing
Reviewed By: ezyang
Differential Revision: D34227616
fbshipit-source-id: 6d1dd0fd8144dfbd9e194cd7564cce017e7db968
(cherry picked from commit e5c1b29fedd5c2a0bad810cedc94aa784136b6aa)
