class_docstr: Dummy communicator that does not communicate at all.
  
  This class is intended to measure the overhead of packing and unpacking.
  This class does not pass the tests.
code: def multi_node_mean_grad(self, model, zero_fill=False):
      params = _memory_utility.extract_params_set_grad(model, zero_fill)
      itemsize = 4
      n_elems_total = _memory_utility.count_grad_elements(
          params, zero_fill
      )
      n_bytes_total = n_elems_total * itemsize
      self.gpu_buffer_a.assign(n_bytes_total)
  
      self._pack_params_to_buffer(
          params,
          "grad",
          buffer=self.gpu_buffer_a,
          allreduce_grad_dtype=np.float32,
          zero_fill=zero_fill,
      )
  
      self._unpack_params_from_buffer(
          params,
          "grad",
          buffer=self.gpu_buffer_a,
          allreduce_grad_dtype=np.float32,
          zero_fill=zero_fill,
      )
  
imports: from chainermn.communicators import _memory_utility
  from chainermn.communicators import mpi_communicator_base
  import numpy
label_desc: Use image node_id 2 for calling the DummyCommunicator obj's underlying member method code with example usage: obj.multi_node_mean_grad(model, zero_fill) without return types