code: def _maybe_download_corpus(tmp_dir):
    
    corpus_url = ("http://www.statmt.org/lm-benchmark/"
                  "1-billion-word-language-modeling-benchmark-r13output.tar.gz")
    corpus_filename = os.path.basename(corpus_url)
    corpus_filepath = os.path.join(tmp_dir, corpus_filename)
    if not os.path.exists(corpus_filepath):
      generator_utils.maybe_download(tmp_dir, corpus_filename, corpus_url)
      with tarfile.open(corpus_filepath, "r:gz") as corpus_tar:
        corpus_tar.extractall(tmp_dir)
docs: Download and unpack the corpus.
  
    Args:
      tmp_dir: directory containing dataset.