This paper considers the optimization problem of the form $\min_{{\bf
x}\in{\mathbb R}^d} f({\bf x})\triangleq \frac{1}{n}\sum_{i=1}^n f_i({\bf x})$,
where $f(\cdot)$ satisfies the Polyak--{\L}ojasiewicz (PL) condition with
parameter $\mu$ and $\{f_i(\cdot)\}_{i=1}^n$ is $L$-mean-squar