request for multi-batch learning support in Faster R-CNN

[machanic]

the light head https://github.com/zengarden/light_head_rcnn
already support multi-batch learning (Light Head largely follows the design of Faster R-CNN and R-FCN)
You can checkout how to implement multi-batch code in Faster R-CNN

[yuyu2172]

OK. I made another PR that summarizes detection model.
#603