add backward test
add backward test

apache/incubator-singa,apache/incubator-singa,nudles/incubator-singa,apache/incubator-singa,apache/incubator-singa,nudles/incubator-singa,nudles/incubator-singa,nudles/incubator-singa,nudles/incubator-singa,apache/incubator-singa