* 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__`

* refine bbox_normalize in infer.py

* add is_bbox_normalize

* rename _forward to build

* check callable

* Rename object_detection to PaddleDetection
* Small fix for doc