could you explain the purpose make the n=1 fixed I want to call the vllm worker directly, but I can not get multipe choice. I checked the code, find this in generate_stream method sampling_params = SamplingParams( n=1, temperature=temperature, top_p=top_p, use_beam_search=use_beam_search, stop=list(stop), stop_token_ids=stop_token_ids, max_tokens=max_new_tokens, top_k=top_k, presence_penalty=presence_penalty, frequency_penalty=frequency_penalty, best_of=best_of, )