Creates hook to stop if the given metric is lower than the threshold.

Usage example:

estimator = ...
# Hook to stop training if loss becomes lower than 100.
hook = early_stopping.stop_if_lower_hook(estimator, "loss", 100)
train_spec = tf.estimator.TrainSpec(..., hooks=[hook])
tf.estimator.train_and_evaluate(estimator, train_spec, ...)

Caveat: Current implementation supports early-stopping both training and evaluation in local mode. In distributed mode, training can be stopped but evaluation (where it's a separate job) will indefinitely wait for new model checkpoints to evaluate, so y