align llm_engine and async_engine. (#1081)

vllm/engine/async_llm_engine.py

@@ -183,10 +183,9 @@ class _AsyncLLMEngine(LLMEngine):
         and updates the scheduler with the model outputs. Finally, it decodes
         the sequences and returns the newly generated results.
         """
-        (seq_group_metadata_list, scheduler_outputs,
-         early_return) = self._schedule()
-        if early_return is not None:
-            return early_return
+        seq_group_metadata_list, scheduler_outputs, ignored = self._schedule()
+        if scheduler_outputs.is_empty():
+            return ignored
 
         # Execute the model.
         output = await self._run_workers_async(
@@ -197,7 +196,7 @@ class _AsyncLLMEngine(LLMEngine):
             blocks_to_copy=scheduler_outputs.blocks_to_copy,
         )
 
-        return self._process_model_outputs(output, scheduler_outputs)
+        return self._process_model_outputs(output, scheduler_outputs) + ignored
 
     async def _run_workers_async(
         self,