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.

transformers/commands/convert.py

@@ -1,7 +1,6 @@
 from argparse import ArgumentParser, Namespace
 from logging import getLogger
 
-from transformers import AutoModel, AutoTokenizer
 from transformers.commands import BaseTransformersCLICommand