Add runtime root path to relative vaes and other submodels (#3631)

This PR fixes a crash that would occur when VAEs and other submodels
have a relative path in the config file.

Author: lstein

invokeai/backend/model_management/model_manager.py

@@ -423,14 +423,15 @@ def get_model(
         if submodel_type is not None and hasattr(model_config, submodel_type):
             override_path = getattr(model_config, submodel_type)
             if override_path:
-                model_path = override_path
+                model_path = self.app_config.root_path / override_path
                 model_type = submodel_type
                 submodel_type = None
                 model_class = MODEL_CLASSES[base_model][model_type]
 
         # TODO: path
         # TODO: is it accurate to use path as id
         dst_convert_path = self._get_model_cache_path(model_path)
+
         model_path = model_class.convert_if_required(
             base_model=base_model,
             model_path=str(model_path), # TODO: refactor str/Path types logic