code: def test_backward_gpu(self):
      if self.out_len == 1:
          self.check_backward(
              cuda.to_gpu(self.x),
              cuda.to_gpu(self.y),
              cuda.to_gpu(self.gz0),
          )
      elif self.out_len == 2:
          self.check_backward(
              cuda.to_gpu(self.x),
              cuda.to_gpu(self.y),
              cuda.to_gpu(self.gz0),
              cuda.to_gpu(self.gz1),
          )
  
imports: import functools
  import unittest
  import numpy
  import six
  import chainer
  from chainer import cuda
  from chainer import functions
  from chainer import gradient_check
  from chainer import links
  from chainer import testing
  from chainer.testing import attr
  from chainer import variable
label_desc: Use image node_id 6 for calling the TestForget obj's underlying member method code with example usage: obj.test_backward_gpu() without return types