a few more comments on dispatch key computation methods (#46128)
Summary: Pull Request resolved: https://github.com/pytorch/pytorch/pull/46128
Test Plan: Imported from OSS
Reviewed By: ailzhang
Differential Revision: D24233868
Pulled By: bhosmer
fbshipit-source-id: efb80fb25d4e3ece3ef9190ee1ed834dff505d7c
