[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

vllm/attention/selector.py

 import enum
+import os
 from functools import lru_cache
 from typing import Type
 
@@ -10,6 +11,8 @@ from vllm.utils import is_cpu, is_hip
 
 logger = init_logger(__name__)
 
+VLLM_ATTENTION_BACKEND = "VLLM_ATTENTION_BACKEND"
+
 
 class _Backend(enum.Enum):
     FLASH_ATTN = enum.auto()
@@ -75,4 +78,10 @@ def _which_attn_to_use(dtype: torch.dtype) -> _Backend:
             "Cannot use FlashAttention backend because the flash_attn package "
             "is not found. Please install it for better performance.")
         return _Backend.XFORMERS
+
+    backend_by_env_var = os.getenv(VLLM_ATTENTION_BACKEND)
+    if backend_by_env_var is not None:
+        return _Backend[backend_by_env_var]
+
+    # Default case.
     return _Backend.FLASH_ATTN