code: def check_backward(self, a_data, p_data, n_data, gy_data):
      def f(a, p, n):
          return functions.triplet(
              a, p, n, margin=self.margin, reduce=self.reduce
          )
  
      gradient_check.check_backward(
          f,
          (a_data, p_data, n_data),
          gy_data,
          dtype=numpy.float64,
          **self.check_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 6 for calling the TestTriplet obj's underlying member method code with example usage: obj.check_backward(a_data, p_data, n_data, gy_data) and returns: functions