[Bugfix] Sets is_first_step_output for TPUModelRunner (#9202)

Author: Allen Wang

vllm/worker/tpu_model_runner.py

@@ -541,7 +541,8 @@ def execute_model(
                         seq_group_metadata_list=ctx.seq_group_metadata_list,
                         scheduler_outputs=ctx.scheduler_outputs,
                         is_async=False,
-                        is_last_step=False)
+                        is_last_step=False,
+                        is_first_step_output=i == 0)
                     model_input.async_callback()
             if use_async_out_proc:
                 return [sampler_outputs[-1]]