[Attention] Refactor CUDA attention backend selection logic (#24794)

Signed-off-by: Matthew Bonanni <mbonanni@redhat.com>
Signed-off-by: Matthew Bonanni <mbonanni001@gmail.com>
Co-authored-by: Luka Govedič <ProExpertProg@users.noreply.github.com>

tests/v1/attention/utils.py

@@ -8,7 +8,7 @@ import pytest
 import torch
 
 from vllm.attention.backends.abstract import AttentionImpl
-from vllm.attention.backends.registry import _Backend, backend_to_class_str
+from vllm.attention.backends.registry import AttentionBackendEnum
 from vllm.config import (
     CacheConfig,
     CompilationConfig,
@@ -20,7 +20,6 @@ from vllm.config import (
     VllmConfig,
 )
 from vllm.config.model import ModelDType
-from vllm.utils.import_utils import resolve_obj_by_qualname
 from vllm.v1.attention.backends.utils import (
     AttentionMetadataBuilder,
     CommonAttentionMetadata,
@@ -120,15 +119,14 @@ def create_common_attn_metadata(
 
 
 def try_get_attention_backend(
-    backend: _Backend,
+    backend: AttentionBackendEnum,
 ) -> tuple[type[AttentionMetadataBuilder], type[AttentionImpl]]:
     """Try to get the attention backend class, skipping test if not found."""
-    backend_class_str = backend_to_class_str(backend)
     try:
-        backend_class = resolve_obj_by_qualname(backend_class_str)
+        backend_class = backend.get_class()
         return backend_class.get_builder_cls(), backend_class.get_impl_cls()
     except ImportError as e:
-        pytest.skip(f"{backend_class_str} not available: {e}")
+        pytest.skip(f"{backend.name} not available: {e}")
         raise AssertionError("unreachable") from None