fix: convert torch.__version__ to str when serializing

```py
>>> type(torch.__version__)
<class 'torch.torch_version.TorchVersion'>
```

This causes a YAML error:

```
  File "/home/jz748/anaconda3/lib/python3.10/site-packages/yaml/representer.py", line 231, in represent_undefined
    raise RepresenterError("cannot represent an object", data)
yaml.representer.RepresenterError: ('cannot represent an object', '2.3.1+cu121')
```

Signed-off-by: Jinzhe Zeng <[email protected]>

deepmd/pt/utils/serialization.py

@@ -47,7 +47,7 @@ def serialize_from_file(model_file: str) -> dict:
     model_dict = model.serialize()
     data = {
         "backend": "PyTorch",
-        "pt_version": torch.__version__,
+        "pt_version": str(torch.__version__),
         "model": model_dict,
         "model_def_script": model_def_script,
         "@variables": {},