docs: def download(data_dir):
    """Download census data if it is not already present."""
    tf.gfile.MakeDirs(data_dir)
  
    training_file_path = os.path.join(data_dir, TRAINING_FILE)
    if not tf.gfile.Exists(training_file_path):
      _download_and_clean_file(training_file_path, TRAINING_URL)
  
    eval_file_path = os.path.join(data_dir, EVAL_FILE)
    if not tf.gfile.Exists(eval_file_path):
      _download_and_clean_file(eval_file_path, EVAL_URL)