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.

templates/adding_a_new_model/tests/modeling_tf_xxx_test.py

@@ -24,7 +24,6 @@ from .utils import CACHE_DIR, require_tf, slow
 
 
 if is_tf_available():
-    import tensorflow as tf
     from transformers.modeling_tf_xxx import (
         TFXxxModel,
         TFXxxForMaskedLM,