Created by: kuke

Simple Usage：

1. Cast specific inputs to float16 (Q: Should we do it automatically?)

  imgs = fluid.layers.cast(images, "float16")

2. Decorate optimizer and minimize scaled loss

 optimizer = fluid.optimizer.Adam(learning_rate=0.001)
	
 mp_optimizer = fluid.contrib.mixed_precision.decorate(
	            optimizer=optimizer, init_loss_scaling=8.0)
	
 scaled_loss, _, _ = mp_optimizer.minimize(avg_cost)

See test_image_classification_fp16.py for details.