code: def _token_to_subtoken_ids(self, token):
      
      cache_location = hash(token) % self._cache_size
      cache_key, cache_value = self._cache[cache_location]
      if cache_key == token:
        return cache_value
  
      ret = _split_token_to_subtokens(
          _escape_token(token, self.alphabet), self.subtoken_to_id_dict,
          self.max_subtoken_length)
      ret = [self.subtoken_to_id_dict[subtoken_id] for subtoken_id in ret]
  
      self._cache[cache_location] = (token, ret)
      return ret
docs: Encode a single token into a list of subtoken ids.