Downloads competition file to output_dir.

def download_file(self, fname, output_dir):
    """Downloads competition file to output_dir."""
    if fname not in self.competition_files:  # pylint: disable=unsupported-membership-test
      raise ValueError("%s is not one of the competition's "
                       "files: %s" % (fname, self.competition_files))
    command = [
        "kaggle",
        "competitions",
        "download",
        "--file",
        fname,
        "--path",
        output_dir,
        "-c",
        self._competition_name,
    ]
    _run_kaggle_command(command, self._competition_name)
    return os.path.join(output_dir, fname)