tidy up code eval_numerical_gradient_b()
tidy up code eval_numerical_gradient_b()

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