class_docstr: Fake for DatasetMetadata which alwasy returns constants.
code: def get_target_class(self, _):
      return 2
  
imports: from __future__ import absolute_import
  from __future__ import division
  from __future__ import print_function
  import unittest
  from six import assertCountEqual
  from eval_lib import classification_results
  from eval_lib import image_batches
  from eval_lib import submissions
  from eval_lib import work_data
  from eval_lib.tests import fake_cloud_client
label_desc: Use image node_id 2 for calling the FakeDatasetMeta obj's underlying member method code with example usage: obj.get_target_class(_) and returns: int