code: def __len__(self):
      return len(self.data)
  
imports: import pickle
  import os
  import random
  import torch
  import numpy
  from torch.utils.data import Dataset, DataLoader
  from .mol_tree import MolTree
  from .jtnn_enc import JTNNEncoder
  from .mpn import MPN
  from .jtmpn import JTMPN
  from .vocab import Vocab
label_desc: Use image node_id 2 for calling the PropDataset obj's underlying member method code with example usage: obj.__len__() and returns: len