better code to employ decaying learning rate

[primetang]

I think tf.train.exponential_decay is a more better and more standard method than sess.run(tf.assign(model.lr, args.learning_rate * (args.decay_rate ** e))) to employ decaying learning rate.

[normanheckscher]

Can implement learning algorithm to learn the best decay rate?
https://arxiv.org/abs/1606.04474