* CUDA kernel for density_prior_box_op.
* Support flatten to 2D.

* wip simplify operator framework

* wip

* wip

* done test=develop

* clean test=develop

* fix test=develop

* fix deps test=develop

* fix cpu build test=develop

* fix tensorrt build test=develop

* fix tests test=develop

* fix test=develop

* fix cpu build test=develop

Density prior box operator for image detection model.

* add detect and end2end code

* update the scale for coodinates restore

* fix merge bug with dev.

* fix merge bug with dev.

* test=develop

* fix code style test=develop

* fix code style test=develop

* test=develop

* test=develop

* test=develop

* Add CUDA implementation for generate_proposals_op.
* Clean code.
* Update code.

* Add roi perspective transform.

* Add roi_perspective_transform_op.

* Fix code style.

* Add python api and fix doc.

* Fix API.spec

* Fix python api.

* Fix API.spec

* Move src to detection.

* Add generate_proposal_labels for Faster-RCNN.

* Add proposals generation operator for Faster-RCNN.

* Add region proposal network (RPN) target assign operator and Python API for Faster-RCNN.

* Add anchor generator operator for Faster-RCNN.
* Add unittest testing.
* Add Python API.

* Add quad transform.

* Fix some syntax error.

* Fix CUDA kernel launch configure.

* Generalize geometry channels.

* Rename QuadTransform to PolygonRestore.

* Rename op.

* Rename op and fix computation.

* Modify CMakeLists.txt for box_restore op.

* Refine code:
1. rename op
2. uncomment unitest on GPU