class_docstr: 读取解析PASCAL VOC2007/2012数据集
code: def __len__(self):
      return len(self.xml_list)
  
file_path: deep-learning-for-image-processing/pytorch_object_detection/faster_rcnn/my_dataset.py
imports: import numpy
  from torch.utils.data import Dataset
  import os
  import torch
  import json
  from PIL import Image
  from lxml import etree
label_desc: Use image node_id 2 for calling the VOCDataSet obj's underlying member method code with example usage: obj.__len__() and returns: len