Fix: add model_def_script to ZBL (#4423)

<!-- This is an auto-generated comment: release notes by coderabbit.ai
-->

## Summary by CodeRabbit

- **New Features**
- Enhanced model definition handling for improved encapsulation and
consistency across different model types.

- **Bug Fixes**
- Ensured that model definition scripts are correctly set to a JSON
string representation for all model instances.

<!-- end of auto-generated comment: release notes by coderabbit.ai -->

deepmd/pt/model/model/__init__.py

@@ -196,7 +196,7 @@ def get_zbl_model(model_params):
     rmax = model_params["sw_rmax"]
     atom_exclude_types = model_params.get("atom_exclude_types", [])
     pair_exclude_types = model_params.get("pair_exclude_types", [])
-    return DPZBLModel(
+    model = DPZBLModel(
         dp_model,
         pt_model,
         rmin,
@@ -205,6 +205,8 @@ def get_zbl_model(model_params):
         atom_exclude_types=atom_exclude_types,
         pair_exclude_types=pair_exclude_types,
     )
+    model.model_def_script = json.dumps(model_params)
+    return model
 
 
 def _can_be_converted_to_float(value) -> Optional[bool]: