code: def test_call_cudnn_backward(self):
      with chainer.using_config("use_cudnn", self.use_cudnn):
          y = self.forward()
          with testing.patch("cupy.cudnn.activation_forward") as func:
              y.backward()
              self.assertEqual(func.called, self.expect)
  
imports: import math
  import unittest
  import numpy
  import six
  import chainer
  from chainer.backends import cuda
  from chainer import functions
  from chainer import gradient_check
  from chainer import testing
  from chainer.testing import attr
  from chainer import utils
label_desc: Use image node_id 3 for calling the TestSigmoidCrossEntropyCudnnCall obj's underlying member method code with example usage: obj.test_call_cudnn_backward() without return types