* add paddle.optimizer.lr.CyclicLR

* add unittest of CyclicLR

* fix code format

* fix bug

* try

* fix CI-Coverage

* fix ValueError

* fix arguments assgin

* fix code format and retry pulling develop to pass ci

* fix typo

* Refactor

* fix function-redefined in test_lr_scheduler.py

* update

* fix conflict

* update

* gamma->exp_gamma

* polish docs

* fix code-style

* adjust code format again

* change format of __all__ in lr.py