[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/dataloading/__init__.py

 """Package for dataloaders and samplers."""
 from .. import backend as F
-from .neighbor_sampler import *
+from . import negative_sampler
+from .base import *
 from .cluster_gcn import *
 from .graphsaint import *
+from .neighbor_sampler import *
 from .shadow import *
-from .base import *
-from . import negative_sampler
-if F.get_preferred_backend() == 'pytorch':
+
+if F.get_preferred_backend() == "pytorch":
     from .dataloader import *
     from .dist_dataloader import *

python/dgl/distgnn/__init__.py

 """
 This package contains DistGNN and Libra based graph partitioning tools.
 """
-from . import partition
-from . import tools
+from . import partition, tools