code: def test_double_backward_cpu(self):
      self.check_double_backward(self.data, self.grads, self.gg)
  
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
  from chainer.utils import type_check
  import chainerx
label_desc: Use image node_id 9 for calling the TestBroadcast obj's underlying member method code with example usage: obj.test_double_backward_cpu() without return types