code: def dialog_turns_to_string(dialog):
      
      # join utterances
      turns_as_strings = map(singe_user_utterances_to_string, dialog)
      # join turns
      return "".join(map(lambda x: x + " " + end_of_turn_symbol + " ", turns_as_strings))
docs: Translates the whole dialog (list of lists) to a string
      :param dialog:
      :return: