code: def check_backward(self, cost_data, xs_data, ys_data, g_data):
      def f(cost, *args):
          xs = args[: len(args) // 2]
          ys = args[len(args) // 2 :]
          return functions.crf1d(cost, xs, ys, reduce=self.reduce)
  
      args = [cost_data] + xs_data + ys_data
      if self.reduce == "mean":
          grad = None
      elif self.reduce == "no":
          grad = g_data
      gradient_check.check_backward(f, args, grad, rtol=1e-3, atol=1e-3)
  
imports: import itertools
  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
label_desc: Use image node_id 6 for calling the TestCRF1d obj's underlying member method code with example usage: obj.check_backward(cost_data, xs_data, ys_data, g_data) and returns: functions