code: def check_backward(self, x_data, y_grad):
      params = [self.link.linear.W]
      if self.initial_bias is not None:
          params.append(self.link.linear.b)
      gradient_check.check_backward(
          self.link, x_data, y_grad, params, atol=1e-2
      )
  
imports: import unittest
  import numpy
  import six
  import chainer
  from chainer.backends import cuda
  from chainer import gradient_check
  from chainer.initializers import constant
  from chainer import links
  from chainer import testing
  from chainer.testing import attr
  from chainer.testing import condition
  from chainer.utils import type_check
label_desc: Use image node_id 5 for calling the TestMaxout obj's underlying member method code with example usage: obj.check_backward(x_data, y_grad) without return types