update mnist project based on mlflow2.7.1

[Frankkk2000]

Some changes regarding to mlflow2.7.1, For example: convert_png_to_mlflow_json.py, mlflow2.7.1 needs a new format mlflow_v2_dict = {"dataframe_split":dct}

https://mlflow.org/docs/latest/models.html#local-model-deployment