[Misc] Black auto fix. (#4680)

* [Misc] Black auto fix.

* fix pylint disable
Co-authored-by: Steve <ubuntu@ip-172-31-34-29.ap-northeast-1.compute.internal>

python/dgl/backend/tensorflow/__init__.py

 import os
-os.environ['TF_FORCE_GPU_ALLOW_GROWTH'] = 'true'
 
-from .tensor import *
+os.environ["TF_FORCE_GPU_ALLOW_GROWTH"] = "true"
+
 from .sparse import *
+from .tensor import *

python/dgl/backend/tensorflow/sparse_optim.py

-"""Sparse optimizer is not supported for tensorflow"""
\ No newline at end of file
+"""Sparse optimizer is not supported for tensorflow"""