Only retain the best metric model, and delete the others

[gregtatum]

It costs money to store models in the cloud. We could save a bit, and make the output of the train tasks a bit less confusing if we just stored a single final model. As far as I have seen, we never use any of the other models unless I'm missing something.

We would have to make sure that training continuation is updated and the fetching of old models for training still works.

What do you think @eu9ene?