Fix validation set shuffling (#481)

Author: elseml

bayesflow/workflows/basic_workflow.py

@@ -923,7 +923,9 @@ def _fit(
             elif isinstance(validation_data, int):
                 raise ValueError(f"No simulator found for generating {validation_data} data sets.")
 
-            validation_data = OfflineDataset(data=validation_data, batch_size=dataset.batch_size, adapter=self.adapter)
+            validation_data = OfflineDataset(
+                data=validation_data, batch_size=dataset.batch_size, adapter=self.adapter, stage="validation"
+            )
             monitor = "val_loss"
         else:
             monitor = "loss"
@@ -982,6 +984,8 @@ def _on_training_finished(self):
             else:
                 file_ext = self.checkpoint_name + ".keras"
 
-            logging.info(f"""Training is now finished.
+            logging.info(
+                f"""Training is now finished.
             You can find the trained approximator at '{self.checkpoint_filepath}/{self.checkpoint_name}.{file_ext}'.
-            To load it, use approximator = keras.saving.load_model(...).""")
+            To load it, use approximator = keras.saving.load_model(...)."""
+            )