code: def __call__(self, key, value):
      self.target[key] = value
      return self.target[key]
  
imports: import chainer
  import chainer.testing
  import chainer.testing.attr
  import chainermn
  from chainermn.iterators.multi_node_iterator import _build_ctrl_msg
  from chainermn.iterators.multi_node_iterator import _parse_ctrl_msg
  import numpy
  import platform
  import pytest
  from six.moves import range
  import unittest
label_desc: Use image node_id 3 for calling the DummySerializer obj's underlying member method code with example usage: obj.__call__(key, value) and returns: self