code: def __len__(self):
      return self._length
  
imports: import os
  import logging
  import sys
  import re
  import mxnet
  from common import TemporaryDirectory
  from mxnet import np
  from mxnet.gluon import nn, loss
  from mxnet.gluon.contrib.estimator import estimator, event_handler
  from mxnet.gluon.contrib.estimator.event_handler import LoggingHandler
  from mxnet.gluon.contrib.estimator.event_handler import ValidationHandler
  from mxnet.gluon.contrib.estimator import EpochEnd
  from mxnet.gluon.data.dataset import Dataset
label_desc: Use image node_id 3 for calling the AxisArrayDataset obj's underlying member method code with example usage: obj.__len__() and returns: self