Allow forcing single-threaded execution at runtime.
Allow forcing single-threaded execution at runtime.

Summary: Might be useful for the EXC_RESOURCE / CPU issues.

Reviewed By: salexspb

Differential Revision: D4565494

fbshipit-source-id: 74ac9edeba6334a46ee6799a93ca96eb68216439

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