[platform] fix attn backend for cuda

Signed-off-by: Mengqing Cao <cmq0113@163.com>

Author: MengqingCao

tests/kernels/test_attention_selector.py

@@ -4,14 +4,21 @@
 import torch
 
 from tests.kernels.utils import override_backend_env_variable
-from vllm.attention.selector import get_attn_backend
+from vllm.attention.selector import _cached_get_attn_backend, get_attn_backend
 from vllm.platforms.cpu import CpuPlatform
 from vllm.platforms.cuda import CudaPlatform
 from vllm.platforms.openvino import OpenVinoPlatform
 from vllm.platforms.rocm import RocmPlatform
 from vllm.utils import STR_FLASH_ATTN_VAL, STR_INVALID_VAL
 
 
+@pytest.fixture(autouse=True)
+def clear_cache():
+    """Clear lru cache to ensure each test case runs without caching.
+    """
+    _cached_get_attn_backend.cache_clear()
+
+
 @pytest.mark.parametrize(
     "name", ["TORCH_SDPA", "ROCM_FLASH", "XFORMERS", "FLASHINFER", "OPENVINO"])
 @pytest.mark.parametrize("device", ["cpu", "openvino", "hip", "cuda"])
@@ -39,10 +46,12 @@ def test_env(name: str, device: str, monkeypatch):
                                        False)
         assert backend.get_name() == "OPENVINO"
     else:
-        with patch("vllm.attention.selector.current_platform", CudaPlatform()):
-            backend = get_attn_backend(16, torch.float16, torch.float16, 16,
-                                       False)
-        assert backend.get_name() == name
+        if name in ["XFORMERS", "FLASHINFER"]:
+            with patch("vllm.attention.selector.current_platform",
+                       CudaPlatform()):
+                backend = get_attn_backend(16, torch.float16, torch.float16,
+                                           16, False)
+            assert backend.get_name() == name
 
 
 def test_flash_attn(monkeypatch):

vllm/platforms/cuda.py

@@ -154,8 +154,7 @@ def get_attn_backend_cls(cls, selected_backend, head_size, dtype,
             logger.info("Using XFormers backend.")
             return "vllm.attention.backends.xformers.XFormersBackend"
         elif selected_backend == _Backend.FLASH_ATTN:
-            logger.info("Using FlashAttention backend.")
-            return "vllm.attention.backends.flash_attn.FlashAttentionBackend"
+            pass
         elif selected_backend:
             raise ValueError(
                 f"Invalid attention backend for {cls.device_name}")