code: def check_backward(self, x_data, W_data, y_grad):
      if W_data is None:
          params = [self.link.W]
          gradient_check.check_backward(
              self.link, x_data, y_grad, params, atol=1e-2
          )
      else:
          gradient_check.check_backward(
              self.link, (x_data, W_data), y_grad, atol=1e-2
          )
  
imports: import unittest
  import numpy
  import chainer
  from chainer.backends import cuda
  from chainer import gradient_check
  from chainer import links
  from chainer import testing
  from chainer.testing import attr
  from chainer.testing import condition
label_desc: Use image node_id 6 for calling the TestScale obj's underlying member method code with example usage: obj.check_backward(x_data, W_data, y_grad) without return types