Fix LabelCrossEntropyOp to ENFORCE >= 0 as well.
Fix LabelCrossEntropyOp to ENFORCE >= 0 as well.

Summary: As desc.

Differential Revision: D4570755

fbshipit-source-id: 665326da7a057e357a31736a0c196dccc83f4ccc

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