mpi const cast
mpi const cast

Summary: This fixes https://github.com/caffe2/caffe2/issues/160

Reviewed By: pietern

Differential Revision: D4617278

fbshipit-source-id: 6fbc7727d62915cfe0426b528d707756580e7b78

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