docstr: Returns the number of documents. 
docstr_tok: ["Returns","the","number","of","documents","."]
code: def __len__(self):
      """Returns the number of documents."""
      return len(self.documents)
  
imports: import re
  import torch
  import torch.nn
  import torch.nn.functional
  from torch.utils.data import Dataset, DataLoader, random_split
  from tqdm import tqdm
  from reformer_pytorch import Reformer, ReformerLM
  from transformers import BertTokenizer, PreTrainedTokenizer
  from fairseq.optim.adafactor import Adafactor
  import os
  import json
  import logging
  from datetime import datetime
label_desc: Use image node_id 2 for calling the WikiDataset obj's underlying member method code with example usage: obj.__len__() and returns: len