Add Cyclic LR callback

[Avsecz]

https://github.com/bckenstler/CLR/blob/master/clr_callback.py

Paper: https://arxiv.org/abs/1506.01186

[Avsecz]

also, the learnining-rate finder could be easy to implement as a function: https://techburst.io/improving-the-way-we-work-with-learning-rate-5e99554f163b