Prove that the variance of the stochastic gradient with a mini-batch size of $S^{(i)}$ is equal to the variance with a mini-batch size of 1 divided by $S^{(i)}$, i.e., $v^{(i)}=$ $\frac{1}{S^{(i)}}$ $v_{s^{(i)=1}}^{(i)}$.
$v$ and $S$ denotes the variance for a mini-batch of size S