keras/src/optimizers/schedules/learning_rate_schedule.py/CosineDecay/_warmup_function
class CosineDecay:    def _warmup_function(
        self, step, warmup_steps, warmup_target, initial_learning_rate
    ):
        with ops.name_scope(self.name):
            completed_fraction = step / warmup_steps
            total_step_delta = warmup_target - initial_learning_rate
            return total_step_delta * completed_fraction + initial_learning_rate