Replace val_loss with monitor_metric
Validation epoch end has a special key called 'val_loss' which enables early stopping and checkpoint.
However, a better name is likely monitor_metric
    def validation_epoch_end(self, outputs):
        return {'monitor_metric': whatever_thing_i_want}

This makes it clear to the user that they can pass in anything (epoch, loss, accuracy, bleu, etc).
Obviously this change needs to be backward compatible