功能：用FR共轭梯度法求解无约束问题：min f(x) 输入：x0是初始点，fun,gfun分别是目标函数和梯度 输出：x,val分别是近似最优点和最优值，k是迭代次数-Features: use FR conjugate gradient method to solve the unconstrained problem: min f (x) input: x0 is the initial point, fun, gfun is the objective functio