Fix F401 flake8 warning (x88 / 116).

This change is mostly autogenerated with:

    $ python -m autoflake --in-place --recursive --remove-all-unused-imports --ignore-init-module-imports examples templates transformers utils hubconf.py setup.py

I made minor changes in the generated diff.

examples/distillation/scripts/extract.py

@@ -20,7 +20,7 @@ import argparse
 
 import torch
 
-from transformers import BertForMaskedLM, GPT2LMHeadModel, RobertaForMaskedLM
+from transformers import GPT2LMHeadModel, RobertaForMaskedLM
 
 
 if __name__ == "__main__":