code: def test_len(self):
      self.assertEqual(len(self.dataset), 2)
  
imports: import os
  import pickle
  import unittest
  import numpy
  from chainer import datasets
  from chainer.datasets import image_dataset
  from chainer import testing
label_desc: Use image node_id 2 for calling the TestLabeledImageDataset obj's underlying member method code with example usage: obj.test_len() without return types