Add group collector limit and add option for enable sum loss
Add group collector limit and add option for enable sum loss

Summary: as title. Add num of examples limit for group collect. Add option for enabling sum loss in BatchLRLoss

Reviewed By: xianjiec

Differential Revision: D4602311

fbshipit-source-id: 5b2a244f1f0e9f1ab0f4590e94828fd18d018d8d

bwasti/caffe2,bwasti/caffe2,sf-wind/caffe2,Yangqing/caffe2,bwasti/caffe2,xzturn/caffe2,pietern/caffe2,sf-wind/caffe2,sf-wind/caffe2,davinwang/caffe2,davinwang/caffe2,caffe2/caffe2,bwasti/caffe2,xzturn/caffe2,Yangqing/caffe2,pietern/caffe2,Yangqing/caffe2,davinwang/caffe2,pietern/caffe2,xzturn/caffe2,Yangqing/caffe2,davinwang/caffe2,davinwang/caffe2,sf-wind/caffe2,pietern/caffe2,bwasti/caffe2,Yangqing/caffe2,xzturn/caffe2,sf-wind/caffe2,xzturn/caffe2,pietern/caffe2