code: def backward(self, target_input_indexes, grad_outputs):
      return self._mock_backward(target_input_indexes, grad_outputs)
  
imports: import unittest
  import mock
  import numpy
  import six
  import chainer
  from chainer import _backprop_utils
  from chainer.backends import cuda
  from chainer import testing
  from chainer.testing import attr
label_desc: Use image node_id 1 for calling the FuncWithBackward obj's underlying member method code with example usage: obj.backward(target_input_indexes, grad_outputs) and returns: self