[multi-step] Raise error if not using async engine (#7703)

vllm/engine/llm_engine.py

@@ -1302,6 +1302,11 @@ class LLMEngine:
             raise NotImplementedError(
                 "Pipeline parallelism is only supported through AsyncLLMEngine "
                 "as performance will be severely degraded otherwise.")
+
+        if self.scheduler_config.num_scheduler_steps > 1:
+            raise NotImplementedError(
+                "Multiple scheduler steps (multi-step) are only supported "
+                "through AsyncLLMEngine. ")
         seq_group_metadata_list, scheduler_outputs = self.scheduler[
             0].schedule()