Add support for creating inference-only SageMaker pipelines

[thvasilo]

Currently inference tasks in our SageMaker pipeline generator always get their model input path by a variable model_output_path that gets set during the creation of a training task.

We should add an optional CLI argument that allows users to provide their own path for model artifacts, which will make it possible to define inference-only pipelines with job sequences like gconstruct inference or gsprocessing dist_part inference.