Converts list of int subtokens ids into a string.

def decode(self, subtokens):
    """Converts list of int subtokens ids into a string."""
    if isinstance(subtokens, np.ndarray):
      # Note that list(subtokens) converts subtokens to a python list, but the
      # items remain as np.int32. This converts both the array and its items.
      subtokens = subtokens.tolist()

    if not subtokens:
      return ""

    assert isinstance(subtokens, list) and isinstance(subtokens[0], int), (
        "Subtokens argument passed into decode() must be a list of integers.")

    return _unicode_to_native(
        _join_tokens_to_string(self._subtoken_ids_to_tokens(subtokens)))