[FEATURE] Add Stopping Criteria to Grid Search

[meganjkurka]

🚀 Feature

Add the ability to stop Grid Search if the validation metric exceeds a certain value or if the models are not getting better (i.e. stop if BLEU hasn’t improved by 1 over the 5 best models).

Motivation

Automatic stopping will ensure that the grid search is stopped once the models are "good enough" and there is no unnecessary waste of resources.