code: def check_backward(self, x, gy):
      y = chainer.Variable(x) + chainer.Variable(x)
      y.grad = gy
      with mock.patch("cupy.cuda.nvtx.RangePush") as push, mock.patch(
          "cupy.cuda.nvtx.RangePop"
      ) as pop:
          with self.h:
              y.backward()
  
      push.assert_called_once_with("_ + _.backward")
      pop.assert_called_once_with()
  
imports: import unittest
  import mock
  import numpy
  import chainer
  from chainer.backends import cuda
  from chainer import function_hooks
  from chainer import testing
  from chainer.testing import attr
label_desc: Use image node_id 6 for calling the TestCUDAProfileHook obj's underlying member method code with example usage: obj.check_backward(x, gy) without return types