[Test] Add xformer and flash attn tests (#3961)

Co-authored-by: Simon Mo <simon.mo@hey.com>

tests/basic_correctness/test_basic_correctness.py

@@ -4,6 +4,8 @@ Run `pytest tests/basic_correctness/test_basic_correctness.py`.
 """
 import pytest
 
+from vllm.attention.selector import VLLM_ATTENTION_BACKEND
+
 MODELS = [
     "facebook/opt-125m",
     "meta-llama/Llama-2-7b-hf",
@@ -14,6 +16,7 @@ MODELS = [
 @pytest.mark.parametrize("dtype", ["half"])
 @pytest.mark.parametrize("max_tokens", [5])
 @pytest.mark.parametrize("enforce_eager", [False, True])
+@pytest.mark.parametrize("attn_backend", ["XFORMERS", "FLASH_ATTN"])
 def test_models(
     hf_runner,
     vllm_runner,
@@ -22,7 +25,10 @@ def test_models(
     dtype: str,
     max_tokens: int,
     enforce_eager: bool,
+    attn_backend: str,
+    monkeypatch,
 ) -> None:
+    monkeypatch.setenv(VLLM_ATTENTION_BACKEND, attn_backend)
     hf_model = hf_runner(model, dtype=dtype)
     hf_outputs = hf_model.generate_greedy(example_prompts, max_tokens)
     del hf_model