* [CodeStyle][F401] remove unused imports in unittests/dygraph_to_static

* [CodeStyle][F401] remove unused imports in unittests/ir

* add noqa after required imports

* [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

Reduce max iter size to fix windows openblas test_yolov3 random failure.
Decrease batch size to fix pe related unittest random failure.

As the title

Unit test reported memory not enough on CPU machines. Reduce batch size again.

I found the unittest failed due to batch size. Maybe the reason is that our CI machine has limited memory. I decreased the batch size.

As the title

[Dy2stat] decrease the batch size to decrease GPU usage.

* cast var in convert_logical_XX. 

* Add convert_ifelse function in convert_operators.py  

* Add logical_transformer. Remove LogicalTransformer from loop_transformer.py 

* Revert modified tests in PR24799(convert_while_stmt). 

* Comment and modify code that doesn't support `return` statement. 

* Remove unnecessary class: MergeAssignTransformer, NodeTestTransformer and IfConditionVisitor in ifelse_transformer. 

Add Yolov3 as ProgramTranslator Unit Test. The YoloV3 code is referred from PaddlePaddle/models/dygraph/yolov3