Make instance norm grad test less flakey
Make instance norm grad test less flakey

Summary:
Instance norm failed grad check in some cases that needed a smaller step size.  Decreased step size, but also increased threshold slightly.

Related diff: D4627379

Reviewed By: kennyhorror

Differential Revision: D4941827

fbshipit-source-id: d6f565340da92af40bfee90627960a3356c69412

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