class_docstr: Cityscapes Dataloader
code: def _mask_transform(self, mask):
      target = self._class_to_index(np.array(mask).astype("int32"))
      return mx.nd.array(target).astype("int32")
  
file_path: gluon-cv/gluoncv/data/cityscapes.py
imports: import os
  import numpy
  from PIL import Image
  import mxnet
  from .segbase import SegmentationDataset
label_desc: Use image node_id 4 for calling the CitySegmentation obj's underlying member method code with example usage: obj._mask_transform(mask) and returns: mx