fix a bug when provided metric is None
fix a bug when provided metric is None

ijingo/incubator-singa,kaiping/incubator-singa,nudles/incubator-singa,ijingo/incubator-singa,kaiping/incubator-singa,nudles/incubator-singa,nusdbsystem/incubator-singa,apache/incubator-singa,kaiping/incubator-singa,ijingo/incubator-singa,apache/incubator-singa,nusdbsystem/incubator-singa,nusdbsystem/incubator-singa,kaiping/incubator-singa,nusdbsystem/incubator-singa,ijingo/incubator-singa,apache/incubator-singa,nudles/incubator-singa,ijingo/incubator-singa,apache/incubator-singa,nusdbsystem/incubator-singa,nudles/incubator-singa,nudles/incubator-singa,kaiping/incubator-singa,apache/incubator-singa