[Speedster] Make Speedster optimize_model() return InferenceLearner also for StableDiffusion models

[Michele Zanotti]

Speedster optimize_model function should return an object of type BaseInferenceLearner. However when optimizing Stable Diffusion models, the function returns an object of type StableDiffusionPipeline.

It would be useful if the function returned an Inference Learner also for Stable Diffusion to have a common API and expose utility methods such as save() and get_size() on the optimized models.