*   Remove unused dynamic learning rate support.

## Thanks to our Contributors

This release contains contributions from many people at Google, as well as:

(David) Siu-Kei Muk, Ag Ramesh, Anton Dmitriev, Artem Sobolev, Avijit-Nervana,
Bairen Yi, Bruno Goncalves, By Shen, candy.dc, Cheng Chen, Clayne Robison,
coder3101, Dao Zhang, Elms, Fei Hu, feiquan, Geoffrey Irving, Guozhong Zhuang,