* Add license to the top of files

* Add licenses agreement on the demo files

* Adjust the sync_cycle automatically.

* Change the multiplier type to double.

* Adjust the sync_period.

* Fix comment.

* Add the pre-commit

* Run pre-commit with other files.

* Display documentations of demos on the Docs.

* Rename the title from `demos` to `Examples`.

* Provide English tutorial on how to use VisualDL in Keras

* Add the English Tutorial of PyTorch.

* Add the English MXNet tutorial.

* Remove links in the MXNet tutorial.