Method stop

dnn/test/common/timer.h:26–31

        m_start_point = clock::now();
    }
    void stop() {
        megdnn_assert(m_started);
        megdnn_assert(!m_stopped);
        m_stopped = true;
        m_stop_point = clock::now();
    }
    size_t get_time_in_us() const {
        return std::chrono::duration_cast<std::chrono::microseconds>(
                       m_stop_point - m_start_point)