code: def test_ignore_labels(self):
      for _, label_orig in self.dataset:
          H, W = label_orig.shape
          label_out = np.ones((H, W), dtype=np.int32) * -1
          for label in cityscapes_labels:
              label_out[label_orig == label.trainId] = label.id
  
imports: import numpy
  import os
  import shutil
  import tempfile
  import unittest
  from chainer import testing
  from chainer.testing import attr
  from chainercv.datasets.cityscapes.cityscapes_utils import cityscapes_labels
  from chainercv.datasets import CityscapesSemanticSegmentationDataset
  from chainercv.datasets import CityscapesTestImageDataset
  from chainercv.utils import assert_is_semantic_segmentation_dataset
  from chainercv.utils.testing.assertions.assert_is_image import assert_is_image
  from chainercv.utils import write_image
label_desc: Use image node_id 3 for calling the TestCityscapesSemanticSegmentationDataset obj's underlying member method code with example usage: obj.test_ignore_labels() without return types