add eval_numerical_gradient_b()
add eval_numerical_gradient_b()

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