add HPU case for backend_to_string function (#69225)
Summary:
Change-Id: If8ed7f1161343a2e494d8b964576e1ee193007f7
Fixes https://github.com/pytorch/pytorch/issues/65609
Pull Request resolved: https://github.com/pytorch/pytorch/pull/69225
Reviewed By: gchanan
Differential Revision: D32804545
Pulled By: wconstab
fbshipit-source-id: bdf359bd779113153ebdecc515edba94e47e0ae4
