Add no_prefetch option to prefetch_op.
Add no_prefetch option to prefetch_op.

Summary:
We may not want to run the operator in a prefetch manner if we don't need any prefetching.
The option allows without modification to any operator to run it ina normal fashion.

Differential Revision: D6717720

fbshipit-source-id: 10114d68edd95258b823603d8532360120421649

xzturn/caffe2,xzturn/caffe2,Yangqing/caffe2,Yangqing/caffe2,xzturn/caffe2,caffe2/caffe2,Yangqing/caffe2,xzturn/caffe2,Yangqing/caffe2,Yangqing/caffe2,xzturn/caffe2