Introduce training checkpointing

[lukstafi]

I.e. saving a restoring all global state: from Node, NodeUI, Formula, Session, and Random (from the standard library).

[jimandrew412]

Hi,

If you want to save and restore all global state in Node, NodeUI, Formula, Session, and Random, you can use a serialization library like Pickle or JSON. These libraries allow you to convert your objects into a byte stream or a JSON string, which can then be saved and Download to a file or a database. When you want to restore the state, you can read the file or the database and use the deserialization method provided by the library to recreate the objects. Just make sure to be careful with the security implications of saving and restoring global state.