code: def _prepare_completion_request_payload(self, payload):
      payload["messages"] = [
          {"role": "user", "content": payload.pop("prompt")}
      ]
      return payload
  
file_path: mlflow/mlflow/gateway/providers/openai.py
imports: from mlflow.exceptions import MlflowException
  from mlflow.gateway.config import OpenAIAPIType, OpenAIConfig, RouteConfig
  from mlflow.gateway.providers.base import BaseProvider
  from mlflow.gateway.providers.utils import send_request
  from mlflow.gateway.schemas import chat, completions, embeddings
  from mlflow.utils.uri import append_to_uri_path, append_to_uri_query_params
label_desc: Use image node_id 5 for calling the OpenAIProvider obj's underlying member method code with example usage: obj._prepare_completion_request_payload(payload) and returns: payload