[dispatcher] avoid autograd fixup step on non-backend keys (#46135)
Summary: Pull Request resolved: https://github.com/pytorch/pytorch/pull/46135
Test Plan: Imported from OSS
Reviewed By: ailzhang
Differential Revision: D24235974
Pulled By: bhosmer
fbshipit-source-id: 21215b31146673caae904bb82395858419641633
