This CR makes two changes:

1. In old loop_transformer, if a class variable, such as "self.a, foo.bar" is a loop var, the Dy2stat will fail because `def func(self.foo)` is not legal syntax. We support class variable by renaming.

2. After https://github.com/PaddlePaddle/Paddle/pull/22892 is merged, we can support `while x < 10` in dygraph. I enable those tests in corresponding Dy2stat

* Add unitTest for `Tensor==constant` for ifElse in dygraph2static test=develop

* support if logic for Variable in dygraph, test=develop

* fix test_learning_rate_scheduler.py, test=develop

* fix optest, test=develop

* fix error message, test=develop

* fix optest,test=develop

* fix comment, test=develop

This PR does exact the thing as the title. The reason is that we plan to develop 4 decorators

"dygraph_to_static_code"
"dygraph_to_static_program"
"dygraph_to_static_func"
"dygraph_to_static_output"

 The 4 decorators will emphasize different part when translating dygraph to static graph. Decorator name "dygraph_to_static_graph" is too big for the function it implements.

* Support to use external function test=develop

* refine the parms of ast_to_func test=develop

* Support and/or in controlFlow if test=develop

* Support and/or in controlFlow if test=develop

* Refine IsControlFlow interface test=develop

As the title

* Modify the way of inserting newly defined func_nodes test=develop

* change dygraph_to_static_output to dygraph_to_static_graph. test=develop

* Remove duplicate code. test=develop

* Follow comments from Liujie. test=develop

* change dygraph_to_static_output to dygraph_to_static_graph. test=develop

* add is_control_flow_if  test=develop

* support nested if/else

* support to derivate returns the parameter list automatically

* polish tranform function of slice

* fix modify x.numpy()[i] slice function

* support to transform ast.node into callable function

* fix get_name_ids bug and add more unittest test=develop

* fix requirements.txt test=develop

* remove useless import statement test=develop

* Fixed version compatibility issues in param of function test=develop

* use decorater to test ast_to_func test=develop

* add textwrap.dedent for source_code test=develop

* polish code comment

* fix compatibility with python2 and python3 test=develop

* fix gast version error test=develop

* fix gast repo test=develop

* polish transfer_from_node_type code test=develop

* add nested_if_else unittest test=develop

* split IfElseTransformer test=develop

* specify gast version test=develop

* fix ast_to_func root type test=develop

This PR provides very basic and simple framework for transforming Dygraph to Static Graph.

API names, final outputs are not determined yet. Feel free to modify or add class/function/type when you think the framework is not extendable for you.