add comment about possible caching optimization
add comment about possible caching optimization

mitchellstern/dynet,cherryc/dynet,kamigaito/cnn,Abasyoni/dynet,dhgarrette/cnn,dhgarrette/cnn,kamigaito/cnn,Abasyoni/dynet,Peilun-Li/dynet,Peilun-Li/dynet,mitchellstern/dynet,Abasyoni/dynet,dhgarrette/cnn,cherryc/dynet,Peilun-Li/dynet,cherryc/dynet