code: def _maybe_download_corpus(tmp_dir, vocab_type):
      """Download and unpack the corpus.
  
      Args:
        tmp_dir: directory containing dataset.
        vocab_type: which vocabulary are we using.
  
      Returns:
        The list of names of files.
      """
      if vocab_type == text_problems.VocabType.CHARACTER:
          dataset_url = (
              "https://s3.amazonaws.com/research.metamind.io/wikitext"
              "/wikitext-103-raw-v1.zip"
          )
          dir_name = "wikitext-103-raw"
      else:
          dataset_url = (
              "https://s3.amazonaws.com/research.metamind.io/wikitext"
              "/wikitext-103-v1.zip"
          )
          dir_name = "wikitext-103"
  
      fname = os.path.basename(dataset_url)
      compressed_filepath = generator_utils.maybe_download(
          tmp_dir, fname, dataset_url
      )
      zip_ref = zipfile.ZipFile(compressed_filepath, "r")
      zip_ref.extractall(tmp_dir)
      zip_ref.close()
  
      files = os.path.join(tmp_dir, dir_name, "*")
      train_file, valid_file, test_file = None, None, None
      for f in tf.gfile.Glob(files):
          fname = os.path.basename(f)
          if "train" in fname:
              train_file = f
          elif "valid" in fname:
              valid_file = f
          elif "test" in fname:
              test_file = f
  
      assert train_file, "Training file not found"
      assert valid_file, "Validation file not found"
      assert test_file, "Testing file not found"
  
      return train_file, valid_file, test_file
  
imports: from __future__ import absolute_import
  from __future__ import division
  from __future__ import print_function
  import collections
  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 2 for calling a global function with example usage: _maybe_download_corpus(tmp_dir, vocab_type) and returns: train_file,  valid_file,  test_file