Move attention kernels from meta_registrations to fake_impls (#120682)
This PR is mostly just code movement to make the code review easier - AFAIK it should not change any functionality. The final goal is to remove the xfails for some of the test_fake opinfos for these ops. The opinfos are failing because the outputs can have mixed devices - we need to move them to fake_impls first before we can support mixed device returns.
This PR:
* Move the `_meta_registrations.py` implementations to `fake_impls.py`
* Change the function signature from taking explicit named variables to taking `{args, kwargs}` and normalizing them
* Wrap all the returned tensors in FakeTensors
Tests: relying on opinfos. I also checked `test_fake_*` for these tests (by removing x-fails and patching things until they passed) to verify general correctness.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/120682
Approved by: https://github.com/drisspg
