[V1] [Hybrid] Validate compatibility of attention backend batch reordering at init time (#21557)

Signed-off-by: Thomas Parnell <tpa@zurich.ibm.com>

vllm/v1/attention/backends/utils.py

@@ -167,6 +167,10 @@ class AttentionMetadataBuilder(abc.ABC, Generic[M]):
     # Does this backend/builder support CUDA Graphs for attention.
     attn_cudagraph_support: ClassVar[AttentionCGSupport] = \
         AttentionCGSupport.NEVER
+    # Does this backend/builder reorder the batch?
+    # If not, set this to None. Otherwise set it to the query
+    # length that will be pulled into the front of the batch.
+    reorder_batch_threshold: ClassVar[Optional[int]] = None
 
     @abstractmethod
     def __init__(self, kv_cache_spec: AttentionSpec, layer_names: list[str],
@@ -221,14 +225,6 @@ class AttentionMetadataBuilder(abc.ABC, Generic[M]):
     ) -> bool:
         return False
 
-    def reorder_batch(self, input_batch: "InputBatch",
-                      scheduler_output: "SchedulerOutput") -> bool:
-        """
-        This method can reorder the batch if desired by the backend.
-        :return: Has the batch been reordered (default False).
-        """
-        return False
-
 
 @functools.lru_cache
 def get_kv_cache_layout():