Minor fixes to modelbuilder

Author: butterman0

pymc_extras/model_builder.py

@@ -446,6 +446,7 @@ def load(cls, fname: str):
             sampler_config=json.loads(idata.attrs["sampler_config"]),
         )
         model.idata = idata
+        model.is_fitted_ = True
         dataset = idata.fit_data.to_dataframe()
         X = dataset.drop(columns=[model.output_var])
         y = dataset[model.output_var]
@@ -526,6 +527,8 @@ def fit(
             )
             self.idata.add_groups(fit_data=combined_data.to_xarray())  # type: ignore
 
+        self.is_fitted_ = True
+
         return self.idata  # type: ignore
 
     def predict(