Increase the number of DAG execution worker threads.
Increase the number of DAG execution worker threads.

Reviewed By: akyrola

Differential Revision: D5158414

fbshipit-source-id: add377aec5588076db881a2a3750101710f29732

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