[Core] Deprecate xformers (#29262)

Signed-off-by: Roger Wang <hey@rogerw.io>

tests/basic_correctness/test_basic_correctness.py

@@ -74,9 +74,6 @@ def test_models(
     model_executor: str,
     enable_prompt_embeds: bool,
 ) -> None:
-    if backend == "XFORMERS" and model == "google/gemma-2-2b-it":
-        pytest.skip(f"{backend} does not support gemma2 with full context length.")
-
     with monkeypatch.context() as m:
         m.setenv("VLLM_ATTENTION_BACKEND", backend)