class_docstr: Vocabulary extracts set of unique tokens and
  constructs token to index and index to token lookup tables.
class_docstr_tok: ["Vocabulary","extracts","set","of","unique","tokens","and","constructs","token","to","index","and","index","to","token","lookup","tables","."]
code: def __len__(self):
      return len(self.idx_to_token)
  
imports: import collections
  import random
  import re
  import jax
  import jax.numpy
  import tensorflow_datasets
  import objax
  from objax.functional import one_hot
  from objax.zoo.rnn import RNN
label_desc: Use image node_id 2 for calling the Vocabulary obj's underlying member method code with example usage: obj.__len__() and returns: len