code: def __len__(self):
      return self._size
  
file_path: tensorpack/examples/FasterRCNN/common.py
imports: import numpy
  import cv2
  from tensorpack.dataflow import RNGDataFlow
  from tensorpack.dataflow.imgaug import ImageAugmentor, ResizeTransform
label_desc: Use image node_id 2 for calling the DataFromListOfDict obj's underlying member method code with example usage: obj.__len__() and returns: self