Adding UNK to vocab | Changing default params
Adding UNK to vocab | Changing default params

Summary: UNK needs tobe indexed in the vocabulary for validation to work. Default args now result in training loss decreasing.

Reviewed By: urikz

Differential Revision: D4703393

fbshipit-source-id: e4d6ad100daf8392f8ba1e502f9ecf39bb8ce24a

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