Disable sdpa_with_kv_cache for now (#4319)

Summary:
For some reason `sdpa_with_kv_cache` custom op gives wrong result after prefill. This PR disables it and add a couple of eager mode unit tests.

Pull Request resolved: https://github.com/pytorch/executorch/pull/4319

Reviewed By: helunwencser

Differential Revision: D59988495

Pulled By: larryliu0820

fbshipit-source-id: cce45791f6f492fe3ddc39ef2ad4401ea3dfc407