Convert list of int subtoken ids to a list of string tokens.

def _subtoken_ids_to_tokens(self, subtokens):
    """Convert list of int subtoken ids to a list of string tokens."""
    escaped_tokens = "".join([
        self.subtoken_list[s] for s in subtokens
        if s < len(self.subtoken_list)])
    escaped_tokens = escaped_tokens.split("_")

    # All tokens in the vocabulary list have been escaped (see _escape_token())
    # so each token must be unescaped when decoding.
    ret = []
    for token in escaped_tokens:
      if token:
        ret.append(_unescape_token(token))
    return ret