Make openxla and opexla_eval backend show up in list_backends (#107905)
The reason to keep the non-aot(openxla_eval) backend is discussed in https://github.com/pytorch/xla/issues/5430#issuecomment-1683191662.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/107905
Approved by: https://github.com/jansel
