add amp support for custom backend (#96188)
Fixes #ISSUE_NUMBER
1、add amp support for custom backend
2、optimize the file `backend_registration.py`, and rename it with `custom_backend_registration.py`. And then we would register other funcs for custom backend.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/96188
Approved by: https://github.com/bdhirsh
