Add Lazy backend type string (#84228)
As the title suggest, the `Lazy` case was missing the in the `backend_to_string` switch case causing
```
RuntimeError: Unimplemented backend Lazy
```
when called with a lazy backend.
CC: @wconstab @Krovatkin @desertfire
Pull Request resolved: https://github.com/pytorch/pytorch/pull/84228
Approved by: https://github.com/wconstab
