aiplatofrm.log_model() to represent itself under Model section instead of Parameters

Current `aiplatform.log_model()` method allows for model registry into experiment tracking. However, instead of representing model under separate MODEL section, it just adds it under specified name under ARTIFACTS, that I think is not optimal: such as in case of multiple and many artifacts the ease of finding the MODEL parameter is not optimal (lost in the list of multiple artifacts that are itemized under Artifact ID). Instead I suggest the model to be placed in a separate MODEL/MODELS section similar to PARAMETERS and ARTIFACTS; surely MODEL being an artifact, but having it under separate section improves readibility.