code: def multi_node_mean_grad(self, model):
      raise NotImplementedError()
  
file_path: chainer/tests/chainermn_tests/communicator_tests/test_node_aware_communicator_base.py
imports: import collections
  import os
  import unittest
  import mpi4py.MPI
  import pytest
  from chainermn.communicators.mpi_communicator_base import MpiCommunicatorBase
label_desc: Use image node_id 2 for calling the NodeAwareNaiveCommunicator obj's underlying member method code with example usage: obj.multi_node_mean_grad(model) without return types