replace
            for param_group in optimizer.param_groups:
                param_group['lr'] = learning_rate



with



            decay_start = 25000
            A_ = 1e-3
            B_ = 8000
            C_ = 1e-5
            if iteration < decay_start:
                learning_rate = A_
            else:
                iteration_adjusted = iteration - decay_start
                learning_rate = (A_*(e**(-iteration_adjusted/B_))) + C_ # y = Ae^(-x/B) + C
            for param_group in optimizer.param_groups:
                param_group['lr'] = learning_rate