code: def test_backward_gpu(self):
      self.check_backward(
          cuda.to_gpu(self.cost),
          [cuda.to_gpu(x) for x in self.xs],
          [cuda.to_gpu(y) for y in self.ys],
          cuda.to_gpu(self.g),
      )
  
imports: import itertools
  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 8 for calling the TestCRF1d obj's underlying member method code with example usage: obj.test_backward_gpu() without return types