Use get_model_object instead of model.model_sampling in Deep Shrink node

[pamparamm]

PatchModelAddDownscale is accessing model.model_sampling directly instead of using get_model_object function, causing wrong behavior when using nodes like ModelSamplingDiscrete.

This PR contains fix only for PatchModelAddDownscale node. There might also be another places in code where model.model_sampling is being accessed directly, but I'm not sure whether it causes any issues or not.