code: def check_backward(self, x0_data, x1_data, t_data, gy_data):
      def f(x0, x1, t):
          return functions.contrastive(
              x0, x1, t, self.margin, self.reduce
          )
  
      gradient_check.check_backward(
          f,
          (x0_data, x1_data, t_data),
          gy_data,
          dtype="d",
          **self.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 6 for calling the TestContrastive obj's underlying member method code with example usage: obj.check_backward(x0_data, x1_data, t_data, gy_data) and returns: functions