class_docstr: Stanford Sentiment Treebank binary classification problems.
code: def approx_vocab_size(self):
      return 2**14
  
imports: from __future__ import absolute_import
  from __future__ import division
  from __future__ import print_function
  import os
  import zipfile
  from tensor2tensor.data_generators import generator_utils
  from tensor2tensor.data_generators import problem
  from tensor2tensor.data_generators import text_encoder
  from tensor2tensor.data_generators import text_problems
  from tensor2tensor.utils import registry
  import tensorflow.compat.v1
label_desc: Use image node_id 3 for calling the SentimentSSTBinary obj's underlying member method code with example usage: obj.approx_vocab_size() and returns: unknown