* add save best model & fix dcn configs

* add deformable models
* fix dcn v2
* fix log_iter

* fix infer doc and config default

* refine dataset exists

* fix None proc

* Clean up a bit

* Add `__shared__` annotation for shared variables between modules

* Dedup `num_classes` configurations

* Document `__shared__` usage

* Remove some unused variable

* Improve docstring and add comments for `__shared__`

* Rename object_detection to PaddleDetection
* Small fix for doc