fix caffe2 export of panopticfpn
fix caffe2 export of panopticfpn

Reviewed By: alexander-kirillov

Differential Revision: D25706184

fbshipit-source-id: 7289ed9c3b7cbcefa4a862c557031fcd3b33e242

facebookresearch/detectron2,facebookresearch/detectron2,facebookresearch/detectron2