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 __init__(self, mpi_comm):
      super(DummyCommunicator, self).__init__(mpi_comm)
  
      self.gpu_buffer_a = _memory_utility.DeviceMemory()
  
imports: from chainermn.communicators import _memory_utility
  from chainermn.communicators import mpi_communicator_base
  import numpy
label_desc: Use image node_id 1 to create a new DummyCommunicator object from inherited base classes: mpi_communicator_base with example: obj = DummyCommunicator(mpi_comm)