Implement model.trainer callback functions

[cosmicBboy]

Consider a model.callbacks.* API for defining additional behavior that’s meant to occur at some point during the training/prediction. The development should be use-case driven, e.g.

@model.trainer(pre_callback=..., post_callback=...)
def trainer(...):
    ...

Also additional callbacks for model.predict

[aseemk98]

Hi @cosmicBboy, I'd be glad to take this issue!