  This paper has some inconsistent results, i.e., we made some failed claims
because we did some mistakes for using the test criterion for a series.
Precisely, our claims on the convergence rate of $\mathcal{O}(1/t)$ of SGD
presented in Theorem 1, Corollary 1, Theorem 2 and Corollary 2 are wrongly
derived because they are based on Lemma 5. In Lemma 5, we do not correctly use
the test criterion for a series. Hence, the result of Lemma 5 is not valid. We
would like to thank the community for pointing out this mistake!
