code: def check_double_backward(
      self, x_data, t_data, y_grad, x_grad_grad, t_grad_grad
  ):
      delta = 1
      eps = self.double_backward_options["eps"] * 2
      xp = chainer.backend.get_array_module(x_data)
      mask = xp.abs(xp.abs(x_data - t_data) - delta) < eps
      x_data[mask] = 0
      t_data[mask] = 0
  
      def f(x, t):
          return functions.huber_loss(
              x, t, delta=delta, reduce=self.reduce
          )
  
      gradient_check.check_double_backward(
          f,
          (x_data, t_data),
          y_grad,
          (x_grad_grad, t_grad_grad),
          **self.double_backward_options,
      )
  
imports: 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
  from chainer import utils
label_desc: Use image node_id 9 for calling the TestHuberLoss obj's underlying member method code with example usage: obj.check_double_backward(x_data, t_data, y_grad, x_grad_grad, t_grad_grad) and returns: functions