Removed unnecessary CPU mem alloc when using GPU
Removed unnecessary CPU mem alloc when using GPU

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