The sync_cycle concept is confusing. Can we switch to use a timer?

[jetfuel]

sync_cycle is used to determine how often the VisualDL should persist data into the filesystem. Without extensive documentation, our users won't be able to understand how to choose a proper sync_cycle value to see the real-time Scalar update without impacting the training performance.

We should try some different approach to trigger the PersistToDisk(). For example, VisualDL can set a timer of 30 seconds, and it will call PersistToDisk() when the countdown ends.

[jetfuel]

We could just avoid exposing the variable all together. and make the sync_cycle to adjust itself.