Add alias dispatch key DefaultBackend. (#45718)
Summary: Pull Request resolved: https://github.com/pytorch/pytorch/pull/45718
Test Plan: Imported from OSS
Reviewed By: bhosmer
Differential Revision: D24165892
Pulled By: ailzhang
fbshipit-source-id: ed28bf62b7c6320d966fd10b7a44b14efffe2f62
