code: def multi_node_mean_grad_async(self):
      self.communicator._multi_node_mean_grad_async(
          self.communicated_target,
          self.zero_fill,
          self.allreduce_grad_stream,
      )
  
imports: import chainer
  import copy
label_desc: Use image node_id 3 for calling the _DoubleBufferingOptimizer obj's underlying member method code with example usage: obj.multi_node_mean_grad_async() without return types