Created by: PeterPanZH

I hope the flush time can be setted by user. Some time, 15s is tooooooo long. I thought my installation was wrong. I checked it over and over. I only found that the flush time is fixed at 15s. What a bad design....

Originally posted by @liupgd in https://github.com/PaddlePaddle/VisualDL/issues/524#issuecomment-609576922