class_docstr: Dataset used to parallelize the transformation of the dataset via a DataLoader
code: def __len__(self):
      return len(self.dataset)
  
imports: import argparse
  import os
  import shutil
  from scipy import io
  from torch.utils.data import DataLoader
  from tqdm import tqdm
  from vissl.utils.download import download_and_extract_archive, download_url
  from vissl.utils.io import cleanup_dir
label_desc: Use image node_id 3 for calling the StandfordCarsMapper obj's underlying member method code with example usage: obj.__len__() and returns: len