Fix io processor pooling #28273 (#28484)

Signed-off-by: baonudesifeizhai <baonudesifeizhai@gmail.com>

vllm/entrypoints/openai/serving_pooling.py

@@ -4,7 +4,7 @@
 import asyncio
 import json
 import time
-from collections.abc import AsyncGenerator
+from collections.abc import AsyncGenerator, Sequence
 from typing import Final, cast
 
 import jinja2
@@ -122,6 +122,10 @@ class OpenAIServingPooling(OpenAIServing):
                 engine_prompts = await self.io_processor.pre_process_async(
                     prompt=validated_prompt, request_id=request_id
                 )
+                if not isinstance(engine_prompts, Sequence) or isinstance(
+                    engine_prompts, (str, bytes, bytearray)
+                ):
+                    engine_prompts = [engine_prompts]
 
             elif isinstance(request, PoolingChatRequest):
                 error_check_ret = self._validate_chat_template(