code: def __init__(self):
      self._client_to_message_futures: Dict[
          Client, Dict[bytes, asyncio.Future]
      ] = dict()
      self._clients: Dict[Client, asyncio.Task] = dict()
  
file_path: mars/mars/oscar/backends/core.py
imports: import asyncio
  import copy
  import logging
  from typing import Dict, Union
  from ...oscar.profiling import ProfilingData
  from ...utils import Timer
  from ..errors import ServerClosed
  from .communication import Client
  from .message import _MessageBase, ResultMessage, ErrorMessage, DeserializeMessageFailed
  from .router import Router
label_desc: Use image node_id 1 to create a new ActorCaller object  with example: obj = ActorCaller()