free up dispatch key space (in C++) (#69633)
Summary: Pull Request resolved: https://github.com/pytorch/pytorch/pull/69633
Test Plan: Imported from OSS
Reviewed By: albanD
Differential Revision: D33255193
Pulled By: bdhirsh
fbshipit-source-id: 79773e9c15bf4f2f27675121a49ff5ffd1375238
(cherry picked from commit eac0b1300569e035f3de28a1f0fdce03f60bd270)
