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/summarization/configuration_bertabs.py

@@ -14,9 +14,7 @@
 # See the License for the specific language governing permissions and
 # limitations under the License.
 """ BertAbs configuration """
-import json
 import logging
-import sys
 
 from transformers import PretrainedConfig