fix backward test code
fix backward test code

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