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/contrib/run_camembert.py

@@ -1,7 +1,3 @@
-import tarfile
-import urllib.request
-from pathlib import Path
-
 import torch
 
 from transformers.modeling_camembert import CamembertForMaskedLM

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

examples/summarization/configuration_bertabs.py

@@ -14,9 +14,7 @@
 # See the License for the specific language governing permissions and
 # limitations under the License.
 """ BertAbs configuration """
-import json
 import logging
-import sys
 
 from transformers import PretrainedConfig