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