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Solve the following problem using Python, implementing the functions described below, one line at a time
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Description:
 def _maybe_download_corpora(tmp_dir):
 """Download corpora for multinli. Args: tmp_dir: a string Returns: a string """

  mnli_filename = "MNLI.zip"
  mnli_finalpath = os.path.join(tmp_dir, "MNLI")
  if not tf.gfile.Exists(mnli_finalpath):
    zip_filepath = generator_utils.maybe_download(
        tmp_dir, mnli_filename, _MNLI_URL)
    zip_ref = zipfile.ZipFile(zip_filepath, "r")
    zip_ref.extractall(tmp_dir)
    zip_ref.close()

  return mnli_finalpath