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
  from typing import Set
  import numpy
  from PIL import Image
  from torch.utils.data import DataLoader, Dataset
  from tqdm import tqdm
  from vissl.utils.download import download_and_extract_archive
label_desc: Use image node_id 2 for calling the DSpritesMapper obj's underlying member method code with example usage: obj.__len__() and returns: len