Add numSample field for preComputing
Add numSample field for preComputing

Summary: For customers like Ads, Feeds, MarketPlace, their training data size is super large. It is unnecessary and costly to go over all the data to compute meta information. In this diff, numSample option is added in preCompute, so users have control over how many samples they want to use when computing meta information.

Differential Revision: D4492399

fbshipit-source-id: 7199381d226ee6300a959fc5e116d39984d199fc

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