Add checkpoints during Offline Training

[jbloomAus]

Our PPO models are now stored with checkpoints but our offline trained models aren't. Creating some parity here would be good.

Please ensure:

• [ ] It remains easy to load a model into the app
• [ ] All of the required parameters to instantiate a model are saved with that model
• [ ] Nice utilities facilitate saving/loading of checkpoints as is the case with the PPO models
• [ ] Don't duplicate anything you don't have to with the PPO checkpoint set up.