Replace require_backend with require_backend_is_available (#101891)
[BE] `require_backend_is_available` offers the a more thorough check as `require_backend` but both are often used together. This remove `require_backend` and centralizes on the `require_backend_is_available` decorator
Pull Request resolved: https://github.com/pytorch/pytorch/pull/101891
Approved by: https://github.com/awgu
