Revert to brain_name temporarily

ml-agents/mlagents/trainers/saver/torch_saver.py

             self.policy = module
             self.exporter = ModelSerializer(self.policy)
 
-    def save_checkpoint(self, behavior_name: str, step: int) -> str:
+    def save_checkpoint(self, brain_name: str, step: int) -> str:
-        checkpoint_path = os.path.join(self.model_path, f"{behavior_name}-{step}")
+        checkpoint_path = os.path.join(self.model_path, f"{brain_name}-{step}")
-        self.export(checkpoint_path, behavior_name)
+        self.export(checkpoint_path, brain_name)
-    def export(self, output_filepath: str, behavior_name: str) -> None:
+    def export(self, output_filepath: str, brain_name: str) -> None:
         if self.exporter is not None:
             self.exporter.export_policy_model(output_filepath)