* (feat): add cnplugin cmake

* (feat): add cnplugin deps to backends/CMakeLists.txt and utility.h

* (feat): add box_coder converter

* (feat): add density_prior_box op

* (feat) add multiclass nms converter, while workspace as static tensor

* (ref): change nms test input from file to random, add iou

* (ref): add density_prior_box & box_coder to paddle_use_bridges.h