Add the demo of subgraph splitter

* Enhance reduce op for multi dims.

* Uncomment some unitest.

* Uncomment unitest.

* Remove unused code.

* Fix infershape and python wrapper.

* Add more examples.

* Fix l2_normalize.

* Fix normalization_wrapper.

* Polish code.
1. Rename unitest function.
2. Rename const variable.

1. If all bboxes are not difficult ground truth, the users can not define the data layer for this flag and not the input can be None for detection_map API.
2. Set default value for aspect_ratios in prior_box API.
3. Add more check in prior_box API.

add cpp trainer lib and demo

* Added kernel to is_empty_op

* Added python API

* Updated code as required

* Updated unittests

* "a piece of job."

* "fix typeo"

* "fix ci"

* Dockerfile: unify dev and product docker environment.

* Fix check style error.

* Use envrioment valuable to get version info.

* use latest pip version

* fix ci

* update by comment

* listen_and_serv use localscope

* fix ut