[Attention] Implement universal BACKEND_MAP (#25900)

Signed-off-by: Matthew Bonanni <mbonanni@redhat.com>

vllm/attention/selector.py

@@ -12,7 +12,7 @@ import torch
 
 import vllm.envs as envs
 from vllm.attention.backends.abstract import AttentionBackend
-from vllm.attention.backends.registry import _Backend
+from vllm.attention.backends.registry import _Backend, backend_name_to_enum
 from vllm.logger import init_logger
 from vllm.platforms import current_platform
 from vllm.utils import STR_BACKEND_ENV_VAR, resolve_obj_by_qualname
@@ -20,19 +20,6 @@ from vllm.utils import STR_BACKEND_ENV_VAR, resolve_obj_by_qualname
 logger = init_logger(__name__)
 
 
-def backend_name_to_enum(backend_name: str) -> Optional[_Backend]:
-    """
-    Convert a string backend name to a _Backend enum value.
-
-    Returns:
-    * _Backend: enum value if backend_name is a valid in-tree type
-    * None: otherwise it's an invalid in-tree type or an out-of-tree platform is
-            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]:
     """
     Get the backend override specified by the vLLM attention