code: def test_remove_unused_gpu(self):
      self.link.to_gpu()
      self.check_remove_unused()
  
imports: import numpy
  import unittest
  import chainer
  from chainer.backends.cuda import to_cpu
  from chainer.function import Function
  from chainer import testing
  from chainer.testing import attr
  from chainercv.links import PickableSequentialChain
  from chainercv.utils.testing import ConstantStubLink
label_desc: Use image node_id 4 for calling the TestPickableSequentialChainRemoveUnused obj's underlying member method code with example usage: obj.test_remove_unused_gpu() without return types