code: def test_backward_cpu(self):
      self.check_backward(self.x, self.gy)
  
imports: import unittest
  import numpy
  import chainer
  from chainer.backends import cuda
  from chainer.functions import relu
  from chainer import testing
  from chainermn import create_communicator
  from chainercv.links.model.mobilenet import TFConv2DBNActiv
  from chainercv.utils.testing import attr
label_desc: Use image node_id 6 for calling the TestTFConv2DBNActiv obj's underlying member method code with example usage: obj.test_backward_cpu() without return types