Deprecate training-only attributes on StepperConfig

[jpdunc23]

Adds stepper_training: TrainStepperConfig to the fme.ace training config and removes loss, optimize_last_step_only, n_ensemble, parameter_init, and train_n_forward_steps from StepperConfig.

WARNING: This is a breaking change for existing fme.ace and fme.coupled training configs.

Changes:

• Updates baseline and example ACE configs.

• Renames train_stepper: CoupledTrainStepperConfig to stepper_training: CoupledTrainStepperConfig on the fme.coupled training config.

• Tests added

[oliverwm1]

I'm a bit late on this, but what about something like StepperTrainerConfig for the name? "TrainStepper" is a bit confusing to me, since this isn't really the configuration for a stepper.

[oliverwm1]

Or StepperTrainConfig?

[mcgibbon]

An issue with a different name is, this is explicitly the implementation of the TrainStepperABC generic class, so currently it is a TrainStepper whether that's a confusing name or not.

Another option is to put Adapter in the name, since this is an adapter over the Stepper to make it supply the APIs needed for a TrainStepper. For example, TrainStepperAdapter and TrainStepperAdapterConfig.

Yet another option is to rename Stepper somehow, but idk, TrainStepper really is a Train-api version of the Stepper, and contains a Stepper, so it seems like an appropriate name.

[mcgibbon]

Is the issue with the configuration key train_stepper, with the class name, or both @oliverwm1 ? If the configuration key name is good I would suggest merging and then gathering feedback for a potential rename PR, since the class name is internal and easy to change backwards-compatibly.

[jpdunc23]

I'm hesitant to use "trainer" here since we already use that term when referring to an important class Trainer. I'd suggest stepper_training as an alternative name for the TrainConfig attribute.

From the perspective of Trainer, it's stepper: TrainStepperABC is the same object it always was, so I feel that TrainStepper is an appropriate name. I think it's a nice convention to append Config to the name of the thing being built by the config as in TrainStepperConfig so I would also lean toward keeping the config class name as is.

[mcgibbon]

IMO this is ready to merge, holding off on review in case we need to discuss names

[mcgibbon]

An issue with a different name is, this is explicitly the implementation of the TrainStepperABC generic class, so currently it is a TrainStepper whether that's a confusing name or not.

Another option is to put Adapter in the name, since this is an adapter over the Stepper to make it supply the APIs needed for a TrainStepper. For example, TrainStepperAdapter and TrainStepperAdapterConfig.

Yet another option is to rename Stepper somehow, but idk, TrainStepper really is a Train-api version of the Stepper, and contains a Stepper, so it seems like an appropriate name.

[mcgibbon]

LGTM