code: def check_double_backward(
      self, x0_data, x1_data, t_data, gy_data, gx0_data, gx1_data
  ):
      def f(x0, x1):
          return functions.contrastive(
              x0, x1, t_data, self.margin, self.reduce
          )
  
      gradient_check.check_double_backward(
          f,
          (x0_data, x1_data),
          gy_data,
          (gx0_data, gx1_data),
          dtype="f",
          **self.double_backward_options,
      )
  
imports: import math
  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 TestContrastive obj's underlying member method code with example usage: obj.check_double_backward(x0_data, x1_data, t_data, gy_data, gx0_data, gx1_data) and returns: functions