Run thread pool only on fast cores
Run thread pool only on fast cores

Summary:
choose the number of cores for the thread pool as the number of fast cores

Didn't do any benchmarks, so its mostly FYI diff

Reviewed By: ajtulloch

Differential Revision: D5579797

fbshipit-source-id: 5ada001116c731780f38a62e9c0b500bd64a4bfe

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