SchedulerMixin  
Base class for all schedulers.  
[`SchedulerMixin`] contains common functions shared by all schedulers such as general loading and saving
functionalities.  
[`ConfigMixin`] takes care of storing the configuration attributes (like `num_train_timesteps`) that are passed to
the scheduler's `__init__` function, and the attributes can be accessed by `scheduler.config.num_train_timesteps`.  
Class attributes: