code: def test_mean(self):
      if self.mean is None:
          np.testing.assert_equal(
              self.link.mean, self.link.extractor.mean
          )
      else:
          np.testing.assert_equal(self.link.mean, self.mean)
  
imports: import numpy
  import unittest
  import chainer
  from chainer import testing
  from chainer.testing import attr
  from chainercv.links import FeaturePredictor
label_desc: Use image node_id 4 for calling the TestFeaturePredictor obj's underlying member method code with example usage: obj.test_mean() without return types