Set non-default backend names to lower case (#69400)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/69400
Hopefully this makes naming more consistent. Without this change, some tests will fail for plugins since values can be set to upper case in some cases. This should prevent that and make lookup and comparison consistent.
Test Plan: Check the signals. There is no specific test for this, but all tests should pass.
Reviewed By: mrshenli
Differential Revision: D32836529
fbshipit-source-id: 1b7d2b64e04fe0391b710aa6ed6d1e47df9027a3
