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

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

vllm/model_executor/models/qwen3_vl.py

@@ -43,6 +43,7 @@ from transformers.models.qwen3_vl.video_processing_qwen3_vl import (
     smart_resize as video_smart_resize)
 from transformers.video_utils import VideoMetadata
 
+from vllm.attention.backends.registry import _Backend
 from vllm.attention.layer import check_upstream_fa_availability
 from vllm.compilation.decorators import support_torch_compile
 from vllm.config import VllmConfig
@@ -66,7 +67,6 @@ from vllm.multimodal.processing import (BaseMultiModalProcessor,
                                         PromptReplacement, PromptUpdate,
                                         PromptUpdateDetails)
 from vllm.multimodal.profiling import BaseDummyInputsBuilder
-from vllm.platforms import _Backend
 from vllm.sequence import IntermediateTensors
 from vllm.utils import is_list_of