code: def __len__(self):
      return self.size
  
imports: from __future__ import print_function
  from __future__ import division
  import mxnet
  import numpy
  import gluoncv
  from gluoncv.data.batchify import *
  from gluoncv.data import DetectionDataLoader, RandomTransformDataLoader
label_desc: Use image node_id 2 for calling the DummyDetectionDataset obj's underlying member method code with example usage: obj.__len__() and returns: self