Added statistics for standard deviation
Added statistics for standard deviation

Summary:
Added an exported statistics that helps in computing
standard deviation. It uses an offset-ed mode of computation
to avoid a common pitfall

Reviewed By: azzolini

Differential Revision: D5977811

fbshipit-source-id: e9f3b99a952e10fb3e3eb18a29b5bdca92f82f4c

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