class_docstr: LM1b and IMDb mixed problem class for multitask learning.
code: def vocab_type(self):
      return text_problems.VocabType.CHARACTER
  
file_path: tensor2tensor/tensor2tensor/data_generators/lm1b_imdb.py
imports: from __future__ import absolute_import
  from __future__ import division
  from __future__ import print_function
  from tensor2tensor.data_generators import imdb
  from tensor2tensor.data_generators import lm1b
  from tensor2tensor.data_generators import multi_problem
  from tensor2tensor.data_generators import text_problems
  from tensor2tensor.utils import registry
label_desc: Use image node_id 2 for calling the LanguagemodelLm1bSentimentIMDB obj's underlying member method code with example usage: obj.vocab_type() and returns: text_problems