* support to train in static

* support to independent decorator

* remove in_dygraph_mode condition in ProgramTranslator

* fix import param_guard and add train/eval test=develop

* Modify into ShareVarsFromScope and rm __all__ in partial_program test=develop

* Replace dygraph_to_static_func with @declarative or program_translator.get_func in test_list.py

* Add comments in ConditionalBlock.

* Support list pop last item. 

* Support pop the i-th item. 

* Support an empty tensor array as Input in assign op and set the kernel type is float.

* update dygraph.rnn import

* update dygraph.rnn import

* change unit to cell

* fix math equations

* fix math equations

* fix examples

* remove unused import

* fix examples

* add print transformer & unify print format, test=develop

* remove using of dygraph_to_static_func, test=develop

* remove python stdout capture, test=develop

* fix compatibility problems for PY2, test=develop

* fix detail error, test=develop

* fix type analysis bug, test=develop

* fix print tuple compatible error in PY2, test=develop

* replace get_func to declarative, test=develop

* fix detail bug, test=develop

* fix some detail problems, test=develop

* change visit_call in print transformer, test=develop

* adapt old version pretrain model load, test=develop

* fix infer error & multiple input error, test=develop

* Add trainable_statistics in attr for batch_norm
* Unifying behavior of dynamic graph and static graph

* Simplify code for gast.If in is_control_flow_to_transform.
* Move IsControlFlowVisitor to file utils. 
* Don't use convert_call for build-in func in CallTransformer. 
* Optimize api is_control_flow_to_transform. 
* Polish the document of IsControlFlowVisitor.

* Init complex number neural network, test=develop

* Improve doc writing, test=develop

* Fix elementwise add & sub, test=develop

* Fix elementwise mul act, test=develop

* a) add ut for complex variable; b) remove arg act in elementwise_ops. test=develop

Fix the dygraph mode in matmul, add the support in Linear Op

Fix bug in convert_call because difference exists between python3 and python2. test=develop (#23966)

* [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