Update config_utils.py (#7333)
Fixes this warning:
```
/fsx/qgallouedec/miniconda3/envs/trl/lib/python3.12/site-packages/deepspeed/runtime/config_utils.py:100: PydanticDeprecatedSince211: Accessing the 'model_fields' attribute on the instance is deprecated. Instead, you should access this attribute from the model class. Deprecated in Pydantic V2.11 to be removed in V3.0.
fields = self.model_fields
```
Co-authored-by: Olatunji Ruwase <tjruwase@gmail.com>
