Do not raise error if _Backend is not found

Signed-off-by: wangxiyuan <[email protected]>

vllm/attention/layer.py

@@ -194,11 +194,11 @@ def __init__(
                                         kv_cache_dtype=None,
                                         block_size=16,
                                         is_attention_free=False)
-        attn_backend = backend_name_to_enum(attn_backend.get_name())
-        if attn_backend in {_Backend.FLASH_ATTN, _Backend.FLASH_ATTN_VLLM_V1}:
-            attn_backend = _Backend.XFORMERS
+        backend = backend_name_to_enum(attn_backend.get_name())
+        if backend in {_Backend.FLASH_ATTN, _Backend.FLASH_ATTN_VLLM_V1}:
+            backend = _Backend.XFORMERS
 
-        self.attn_backend = attn_backend if attn_backend in {
+        self.attn_backend = backend if backend in {
             _Backend.TORCH_SDPA, _Backend.XFORMERS
         } else _Backend.TORCH_SDPA
 

vllm/attention/selector.py

@@ -14,16 +14,18 @@
 logger = init_logger(__name__)
 
 
-def backend_name_to_enum(backend_name: str) -> _Backend:
-    assert backend_name is not None
-
-    backend_members = _Backend.__members__
-    if backend_name not in backend_members:
-        raise ValueError(f"Invalid attention backend '{backend_name}'. "
-                         f"Available backends: {', '.join(backend_members)} "
-                         "(case-sensitive).")
+def backend_name_to_enum(backend_name: str) -> Optional[_Backend]:
+    """
+    Convert a string backend name to a _Backend enum value.
 
-    return _Backend[backend_name]
+    Returns:
+    * _Backend: enum value if backend_name is a valid in-tree type
+    * None: otherwise it's an invalid in-tree type or a out-of-tree platform
+            loaded.
+    """
+    assert backend_name is not None
+    return _Backend[backend_name] if backend_name in _Backend.__members__ else \
+          None
 
 
 def get_env_variable_attn_backend() -> Optional[_Backend]: