[Attention] Move Backend enum into registry (#25893)

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

vllm/attention/selector.py

@@ -11,8 +11,9 @@ import torch
 
 import vllm.envs as envs
 from vllm.attention.backends.abstract import AttentionBackend
+from vllm.attention.backends.registry import _Backend
 from vllm.logger import init_logger
-from vllm.platforms import _Backend, current_platform
+from vllm.platforms import current_platform
 from vllm.utils import STR_BACKEND_ENV_VAR, resolve_obj_by_qualname
 
 logger = init_logger(__name__)