remove unsed code and bring back single benchmark mode
remove unsed code and bring back single benchmark mode

Summary: the old gpu single benchmark mode is lost in recent changes. We still need this mode to benchmark some operators. I also removed some unused ancient code

Reviewed By: azzolini

Differential Revision: D5628501

fbshipit-source-id: c5d2c6c99af18c41bead5d86c46a42f05821e2ff

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