code: def check_double_backward(
      self, y_data, z_data, gloss_data, ggy_data, ggz_data
  ):
      def f(y, z):
          return functions.cross_covariance(y, z, self.reduce)
  
      gradient_check.check_double_backward(
          f,
          (y_data, z_data),
          gloss_data,
          (ggy_data, ggz_data),
          **self.double_backward_options,
      )
  
imports: 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
label_desc: Use image node_id 11 for calling the TestCrossCovariance obj's underlying member method code with example usage: obj.check_double_backward(y_data, z_data, gloss_data, ggy_data, ggz_data) and returns: functions