code: def test_call_cudnn_backward(self):
      with chainer.using_config("use_cudnn", self.use_cudnn):
          y = self.forward()
          y.grad = self.gy
          with testing.patch(
              "chainer.backends.cuda.get_cudnn_dropout_states"
          ) as func:
              y.backward()
              assert func.called == (self.use_cudnn == "always")
  
imports: import unittest
  import numpy
  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
label_desc: Use image node_id 4 for calling the TestDropoutCudnnCall obj's underlying member method code with example usage: obj.test_call_cudnn_backward() without return types