[V1][TPU] Add assertion on multi-step-scheduler (#14707)

Signed-off-by: Siyuan Liu <lsiyuan@google.com>

vllm/platforms/tpu.py

@@ -91,13 +91,19 @@ class TpuPlatform(Platform):
         parallel_config = vllm_config.parallel_config
         scheduler_config = vllm_config.scheduler_config
         if parallel_config.worker_cls == "auto":
-            if envs.VLLM_USE_V1:
-                parallel_config.worker_cls = \
-                    "vllm.v1.worker.tpu_worker.TPUWorker"
-            else:
-                if scheduler_config.is_multi_step:
+            if scheduler_config.is_multi_step:
+                if envs.VLLM_USE_V1:
+                    raise NotImplementedError(
+                        "Multi-step scheduling is not supported (and not "
+                        "needed) on vLLM V1. Please launch without "
+                        "--num-scheduler-steps.")
+                else:
                     parallel_config.worker_cls = \
                         "vllm.worker.multi_step_tpu_worker.MultiStepTPUWorker"
+            else:
+                if envs.VLLM_USE_V1:
+                    parallel_config.worker_cls = \
+                        "vllm.v1.worker.tpu_worker.TPUWorker"
                 else:
                     parallel_config.worker_cls = \
                         "vllm.worker.tpu_worker.TPUWorker"