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

* [Cherry-pick Release/2.0] remove redundant visit func, test=develop (#23877)

* [Cherry-pick Release/2.0] [dy2static]Fix a bug of is_dygraph_api and move BasicApiTransformer to a separate file(#23923)

* 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
Co-authored-by: NChen Weihang <chenweihang@baidu.com>

test=release/2.0

* 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

* 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

* support forward hook for dygraph, test=develop

* add optest for forward_hook in dygraph, test=develop

* add optest, test=develop

* polish code, test=develop

* add sample code, test=develop

* rename forwrd_hook to forward_post_hook, test=develop

* fix the api description, test=develop

* fix api description, test=develop

1. Add data_layer_not_check because it is needed in dygraph_to_static where input can be variable size
2. Remove warnings in static analysis because python cannot do exact static analysis