* add enable_imperative, disable_imperative; test=develop

* add unitest; test=develop

* polish example; test=develop

* fix CrossEntropyLoss op en doc, test=develop

* delete unittests,test=release/2.0

* copy api from paddle to paddle.fluid, test=develop

* fix optest, test=develop

* adapt old version pretrain model load, test=develop

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

* Init complex number neural network, test=release/2.0

* Improve doc writing

* Fix elementwise add & sub

* Fix elementwise mul act

* a) add ut for complex variable; b) remove arg act in elementwise_ops

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

chery-pick from the pr#23978

* [Cherry-pick][dy2static]Support recursive call (#23900)
  * Support recursive call.

  * Remove Redundant decorator to pass the Py35 unittest temporarily.

* [Cherry-pick]Fix bug in convert_call because difference exists between python3 and python2. (#23966 )

[Dy2stat] Add Test and Example Code for Different Access to ProgramTranslator and Fix Related Bug (#23958) (#23963)

Cherry pick of 23958

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