Fix F401 flake8 warning (x152 / 268).

This change is mostly autogenerated with:

    $ python -m autoflake --in-place --recursive examples templates transformers utils hubconf.py setup.py

I made minor changes in the generated diff.

examples/run_bertology.py

@@ -22,7 +22,7 @@
 import argparse
 import logging
 import os
-from datetime import datetime, timedelta
+from datetime import datetime
 
 import numpy as np
 import torch