MlflowLogger fail when logging long parameters

[ducthienbui97]

🐛 Bug

Please reproduce using the BoringModel

To Reproduce

Log anything parameters longer than 250 characters

Expected behavior

MlflowLogger not sending parameters longer than 250 characters to mlflow and log warning to user

Environment

• PyTorch Version (e.g., 1.0):
• OS (e.g., Linux):
• How you installed PyTorch (conda, pip, source): pip
• Build command you used (if compiling from source):
• Python version: 3.7
• CUDA/cuDNN version:
• GPU models and configuration:
• Any other relevant information:

Additional context

Mlflow only allow paramters to be at most 500 bytes (250 unicode characters), their limit in database is 250 characters:
https://www.mlflow.org/docs/latest/rest-api.html#log-param
mlflow/mlflow#1976
mlflow/mlflow#3931

[tchaton]

Dear @ducthienbui97,

Thanks for opening a PR.

Best,
T.C