Do not raise error if _Backend is not found

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

vllm/attention/selector.py

@@ -14,14 +14,15 @@
 logger = init_logger(__name__)
 
 
-def backend_name_to_enum(backend_name: str) -> _Backend:
+def backend_name_to_enum(backend_name: str) -> Optional[_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).")
+        logger.warning(f"Invalid attention backend '{backend_name}'. "
+                       f"Available backends: {', '.join(backend_members)} "
+                       "(case-sensitive).")
+        return None
 
     return _Backend[backend_name]