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/run_tf_ner.py

@@ -1,5 +1,4 @@
 # coding=utf-8
-import _pickle as pickle
 import collections
 import datetime
 import glob