improve unsorted segment op speed
improve unsorted segment op speed

Summary:
use cub DeviceReduce, improve the speed from 23k to 26k, but still far
from the 100k, when without dedup.

the bottleneck is at UniqueOp

Reviewed By: harouwu

Differential Revision: D5633828

fbshipit-source-id: e96b8f7317d01c5388c072e7dcfe987abcb01b67

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