Added saving of best checkpoint during early stopping#1859

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sugeeth14 wants to merge 4 commits intoOpenNMT:masterfrom

sugeeth14:best_checkpoint

Commits on Sep 3, 2020

Added saving of best checkpoint during early stopping
Raghavendra Sugeeth P S
committed
Best checkpoint is intialized to None
Raghavendra Sugeeth P S
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1.Validate at every save_checkpoint_steps
Raghavendra Sugeeth P S
committed
Logical fix
Raghavendra Sugeeth P S
committed