fix no logprob

Author: thwu1

rllm/engine/rollout/openai_engine.py

@@ -87,6 +87,7 @@ async def chat_completion(self, messages: list[dict], **kwargs) -> ModelOutput:
                     tool_calls=tool_calls,
                     prompt_ids=[],
                     completion_ids=[],
+                    logprobs=[],
                     prompt_length=prompt_length,
                     completion_length=completion_length,
                     finish_reason=finish_reason,