Returns a set of trial graph config, as a serializable object.
        parameter_id : int

def generate_parameters(self, parameter_id):
        """Returns a set of trial graph config, as a serializable object.
        parameter_id : int
        """
        if len(self.population) <= 0:
            logger.debug("the len of poplution lower than zero.")
            raise Exception('The population is empty')
        pos = -1
        for i in range(len(self.population)):
            if self.population[i].result == None:
                pos = i
                break
        if pos != -1:
            indiv = copy.deepcopy(self.population[pos])
            self.population.pop(pos)
            temp = json.loads(graph_dumps(indiv.config))
        else:
            random.shuffle(self.population)
            if self.population[0].result < self.population[1].result:
                self.population[0] = self.population[1]
            indiv = copy.deepcopy(self.population[0])
            self.population.pop(1)
            indiv.mutation()
            graph = indiv.config
            temp =  json.loads(graph_dumps(graph))
        logger.debug('generate_parameter return value is:')
        logger.debug(temp)
        return temp