add truncation for sparse feature
add truncation for sparse feature

Summary:
truncate id list using the max length computed in compute meta, so that it has determined length,
which is useful for position weighted pooling method.

Reviewed By: sunwael

Differential Revision: D5233739

fbshipit-source-id: f73deec1bb50144ba14c4f8cfa545e1ced5071ce

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