[Refactoring] get_loss/cost/learning_curve()

[eddiebergman]

While doing some small migrations to ruff as part of #77, I noticed that get_loss/cost/learning_curve() are always done on a ConfigResult object, i.e. get_loss(config_result.result, ...). I will just move these to be methods directly on the config object instead of round-abouting to the aforementioned standalone functions. This means any consumers of ConfigResult can just do config_result.loss(...). This is also hiding behind one more abstraction in the BaseOptimizer which essentially wraps these methods with the extra parameters ... filled in.