* [CodeStyle][isort][F401] fix some regression issues

* add import paddle to fix eval call

* Merge branch 'reduce_sum' of https://github.com/GhostScreaming/Paddle into mine_fluid_clean_common.

* Fix some bugs.

* Clean APIs in python/paddle/fluid/layers/control_flow.py

* Polish code style.

* Change API.

* Fix some bugs.

* Fix some bugs.

* remove paddle.fluid.layers.nn.temporal_shift

* code check

* rm unittest

* remove fluid.yolo_box

* remove fluid.yolov3_loss

* change the comments of yolov3_loss to yolo_loss

* isort all files

* revert conflicting files

* revert conflicting files

* revert conflicting files

* revert conflicting files

* revert conflicting files

* remove crop_tensor which is not used in paddle 2.0

* replace the crop_tensor with paddle.crop in orgnizing network

* update config

* re-blacken python code

* temporarily disable date and diff_py_file

* skip a format

* [CodeStyle] remove all future import

* revert test_error.py

* restore future import in example code

* use yapf to format all python file

* yapf exclude two unittests file for they rely on writing and reading file, and format will break them

* disable diff_py_file because too many diff files cause command following failed

* add 2.0 API: device_guard

* Refine the unittest to support py38

    test=develop

* add the support of device index for device_guard.

* add fluid.device_guard to specify the device type for Op