more efficient concat
more efficient concat

yoavg/cnn,kaishengyao/cnn,mitchellstern/dynet,Peilun-Li/dynet,kamigaito/cnn,cherryc/dynet,Peilun-Li/dynet,yanweifu/cnn,cherryc/dynet,dhgarrette/cnn,mrzu/cnn,dhgarrette/cnn,mitchellstern/dynet,Peilun-Li/dynet,Abasyoni/dynet,kaishengyao/cnn,Abasyoni/dynet,kamigaito/cnn,dhgarrette/cnn,cherryc/dynet,Abasyoni/dynet,yoavg/cnn