add center loss
add center loss

ShownX/incubator-mxnet,ShownX/incubator-mxnet,ShownX/incubator-mxnet,ShownX/incubator-mxnet,ShownX/incubator-mxnet,ShownX/incubator-mxnet,ShownX/incubator-mxnet,ShownX/incubator-mxnet,ShownX/incubator-mxnet