code: def _filename(self, iteration):
      # TODO(kuenishi): As a node identifier, should we use node
      # name (e.g. hostname) or MPI rank?
      #
      # hostname is fine when MPI rank changes among same set of nodes.
      # MPI rank is fine when node fails and a new node has come.
      filename = "{:s}.{:d}.{:d}".format(
          self.name, self.comm.rank, iteration
      )
      return filename
  
imports: import errno
  import os
  import shutil
  import tempfile
  import time
  import warnings
  import chainer
  from chainer.training import extension
label_desc: Use image node_id 8 for calling the _MultiNodeCheckpointer obj's underlying member method code with example usage: obj._filename(iteration) and returns: filename