Initialize our optimization params based on our annealing schedule.

def on_train_begin(self, n_epochs:int, epoch:int, **kwargs:Any)->None:
        "Initialize our optimization params based on our annealing schedule."
        res = {'epoch':self.start_epoch} if self.start_epoch is not None else None
        self.start_epoch = ifnone(self.start_epoch, epoch)
        self.tot_epochs = ifnone(self.tot_epochs, n_epochs)
        n = len(self.learn.data.train_dl) * self.tot_epochs
        a1 = int(n * self.pct_start)
        a2 = n-a1
        self.phases = ((a1, annealing_cos), (a2, annealing_cos))
        low_lr = self.lr_max/self.div_factor
        self.lr_scheds = self.steps((low_lr, self.lr_max), (self.lr_max, self.lr_max/self.final_div))
        self.mom_scheds = self.steps(self.moms, (self.moms[1], self.moms[0]))
        self.opt = self.learn.opt
        self.opt.lr,self.opt.mom = self.lr_scheds[0].start,self.mom_scheds[0].start
        self.idx_s = 0
        return res