Revert D34034848: free up dispatch key space (in C++)
Test Plan: revert-hammer
Differential Revision:
D34034848 (https://github.com/pytorch/pytorch/commit/66902560216740db6f38cb7a8458bd29c213f296)
Original commit changeset: 9677ee2c0a1a
Original Phabricator Diff: D34034848 (https://github.com/pytorch/pytorch/commit/66902560216740db6f38cb7a8458bd29c213f296)
fbshipit-source-id: fd50943d915ef813bb9f9ab278fb582429eea3b1
(cherry picked from commit 3acefee1cdb89bc051d1ef2e9deb5698d2bd85c3)
