* [Dy2Stat]Support recursive call.

* Remove Redundant decorator to pass the Py35 unittest temporarily.

[Dy2stat] Add Test and Example Code for Different Access to ProgramTranslator and Fix Related Bug (#23958)

To prepare for publishing APIs, I added tests for that we can access dy2stat through:

@fluid.dygraph.declarative
@fluid.dygraph.jit.declarative
fluid.dygraph.ProgramTranslator()
fluid.dygraph.dygraph_to_static.ProgramTranslator()
fluid.dygraph.dygraph_to_static.program_translator.ProgramTranslator()

It surprised me that we had bugs on those different usages. I have fixed them.

I also added example codes for these new APIs

This PR also pulls my current PR https://github.com/PaddlePaddle/Paddle/pull/23880, so the PR history is long. For reviewer information, you could review this PR after https://github.com/PaddlePaddle/Paddle/pull/23880 is merged

1. Rename Dygraph To Static Decorators to declarative
2. dygraph_to_static_func is still using in some training tests, I cannot delete it now.
3. Add some API docs

* Move BasicApiTransformer to a separate file. test=develop

* Fix a bug: A api in module is not a real dygraph api in dygraph_to_static. test=develop

test=develop

* add cudnn compatiable rnn cell api for dygraph

* update sample code

* update some typos

* fix specify name in param_attr problem

* add pre-commit check

* remove duplicate import, test=develop

* add unittest coverage, test=develop

* make code more tight, test=develop

* cudnn_compatibale -> use_cudnn_impl, test=develop

* change api name, test=develop

* fix if logic in dygraph, test=develop

* fix bug, test=develop

* Spport transform two independed function in dygraph_to_static_output test=develop

* fix unittest error test=develop

* err msg enhance for PRelu

* add ut
test=develop

* err msg enhance for GRUUnit

* add ut
test=develop

* err msg enhance for Embedding

* add ut
test=develop

* err msg enhance for BilinearTensorProduct
test=develop

* rebase dev
test=develop

* add ut
test=develop

* Refine mechanism of calling outer function test=develop

* fix typo test=develop

test=develop

* add instance_norm op for dygraph, test=develop

* add error message, test=develop

* add LayerList insert and extend
test=develop

* add index range check
test=develop

* add sample codes
test=develop

* refine sample code
test=develop

* API(to_variable/save_dygraph) error message enhancement

* error message enhancement for Conv2DTranspose, test=develop

* fix error_message, test=develop

* error message of NCE API enhancement

* fix no_grad signature
test=develop

* check func name instead of doc
test=develop

* Layer add training attr, add Dropout Layer

* add unit test for Dropout
test=develop

* minor fix
test=develop

* add missing args
test=develop

* support global flag in dropout, add docs
test=develop

* minor fix
test=develop

* minor fix
test=develop

* refine code comments
test=develop

* refine Dropout
test=develop

* fix ut
test=develop

* arg expansion
test=develop

* sample code update
test=develop

* prop -> p
test=develop

* fix ut
test=develop

* minor check fix
test=develop

* add Conv/ConvTranspose layers in paddle.nn.layer, test=develop

* add example code in docstring, test=develop

* update unittests to work with test_runner, test=develop

* support no_grad atomic operating
test=develop

* rm create param api
test=develop

* refine sample code
test=develop

* clean impl
test=develop

* static model runner basic implement, test=develop

* add run program op to execute loaded program, test=develop

* refactor static model runner & run program op, test=develop

* reset engine.cc to resolve conflict

* adapt the change of dygraph double grad, test=develop

* refactor impl to solve control flow error, test=develop

* clear debug code, test=develop

* fix ci str compatible error & checkout dygraph grad maker & add example, test=develop

* hide api & add op test, test=develop

* fix run program op test places error, test=develop

* fix program by review comment, test=develop

* delete change var desc name, test=develop

* fix other program by review comment, test=develop

* remove _static_graph_guard, test=develop

* add selectedrows test, test=develop

* remove desc parser, test=develop

* fix detail program, test=develop

* change socpe create & add test, test=develop

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