code: def check_backward(self, x_data, y_grad):
      gradient_check.check_backward(
          distributions.laplace._laplace_cdf,
          x_data,
          y_grad,
          **self.backward_options,
      )
  
imports: import unittest
  import numpy
  from chainer.backends import cuda
  from chainer import distributions
  from chainer import gradient_check
  from chainer import testing
  from chainer.testing import attr
  from chainer import utils
label_desc: Use image node_id 2 for calling the TestLaplaceCDF obj's underlying member method code with example usage: obj.check_backward(x_data, y_grad) without return types