code: def test_to_cpu(self):
      with testing.assert_warns(DeprecationWarning):
          self.sampler.to_gpu()
      with testing.assert_warns(DeprecationWarning):
          self.sampler.to_cpu()
      assert not self.sampler.use_gpu
      assert isinstance(self.sampler._device, backend.CpuDevice)
      self.check_sample()
  
file_path: chainer/tests/chainer_tests/utils_tests/test_walker_alias.py
imports: import unittest
  import numpy
  from chainer import backend
  from chainer.backends import cuda
  from chainer import testing
  from chainer.testing import attr
  from chainer import utils
label_desc: Use image node_id 5 for calling the TestWalkerAlias obj's underlying member method code with example usage: obj.test_to_cpu() without return types