keras/src/optimizers/base_optimizer.py/BaseOptimizer/_backend_increment_gradient_accumulators
class BaseOptimizer:    def _backend_increment_gradient_accumulators(self, grads, acc_grads):
        new_g_accs = [(g + acc_g) for g, acc_g in zip(grads, acc_grads)]
        for n_g_acc, g_acc in zip(new_g_accs, acc_grads):
            g_acc.assign(n_g_acc)