code: def check_backward(self, x_data, y_grad, mask):
      gradient_check.check_backward(
          self.link_wrapper,
          (x_data, mask),
          y_grad,
          (self.link.W, self.link.b),
          dtype="d",
          atol=1e-4,
          rtol=1e-3,
      )
  
imports: import os
  import tempfile
  import unittest
  import numpy
  import chainer
  from chainer.backends import cuda
  from chainer import gradient_check
  from chainer import links
  from chainer.serializers import npz
  from chainer import testing
  from chainer.testing import attr
  from chainer.utils import type_check
label_desc: Use image node_id 6 for calling the TestSimplifiedDropconnectParameterShapePlaceholder obj's underlying member method code with example usage: obj.check_backward(x_data, y_grad, mask) without return types