[BUG] Qwen3-next MTP. Fix attn metadata build bug (vllm-project#26564)

Signed-off-by: Vadim Gimpelson <vadim.gimpelson@gmail.com>
Signed-off-by: bbartels <benjamin@bartels.dev>

Author: vadiklyutiy

vllm/v1/spec_decode/eagle.py

@@ -41,7 +41,6 @@
 from vllm.v1.spec_decode.metadata import SpecDecodeMetadata
 from vllm.v1.utils import CpuGpuBuffer
 from vllm.v1.worker.gpu_input_batch import CachedRequestState, InputBatch
-from vllm.v1.worker.ubatching import dbo_current_ubatch_id
 
 logger = init_logger(__name__)
 
@@ -234,11 +233,11 @@ def propose(
 
         assert self.runner is not None
 
-        # FIXME: need to consider multiple kv_cache_groups
-        ubatch_id = dbo_current_ubatch_id()
-        attn_metadata_builder = self.runner.attn_groups[0][0].metadata_builders[
-            ubatch_id
-        ]
+        if self.attn_metadata_builder is None:
+            attn_metadata_builder = self._get_attention_metadata_builder()
+        else:
+            attn_metadata_builder = self.attn_metadata_builder
+
         attn_metadata = attn_metadata_builder.build_for_drafting(
             common_attn_metadata=common_attn_metadata, draft_index=0
         )
@@ -1076,7 +1075,7 @@ def dummy_run(
                 inputs_embeds=inputs_embeds,
             )
 
-    def _get_attention_metadata_builder(self) -> list[AttentionMetadataBuilder]:
+    def _get_attention_metadata_builder(self) -> AttentionMetadataBuilder:
         """Find and return the attention metadata builders for EAGLE layers.
 
         Returns: