* 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

* Remove Read-Only Basic Type Variable in loop_vars

test=develop

* Put class support in loop for future thing

test=develop

* Modify based on reviewer's comment

test=develop

* expand parameters, test=develop

* support resnet, test=develop

* fix resnet, test=develop

* support duplicable out, test=develop

* support ptb

* fix bugs, test=develop

* support null input, test=develop

* fix bugs, test=develop

* fix batchNorm is_test, test=develop

* refine code, test=develop

* follow comments, test=develop

* follow comments, test=develop

* follow comments, test=develop

* follow comments, test=develop

* Support Parameter type determination test=develop

* support math operator for variable, test=develop

* polish code, test=develop

* polish code, test=develop

* expose dygraph.grad api, test=develop, test=document_fix

* add more parameter in dygraph.grad API, test=develop

* add only_inputs=True parameter, test=develop

* follow comments, test=develop, test=document_fix

* fix typo, test=develop, test=document_fix

1. Add Decorator 'dygraph_to_static_program'
2. Add corresponding ProgramTranslator.get_program
3. Add ProgramTranslator.save_inference_model
4. Modified some warning information of dy2stat
5. Change program cache to contain startup_program because for users who gets program to run, they may like to initialize startup program

* add unittest test=develop

* add function cache test=develop

* Add parameter(learning_rate) in NoamDecay 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.

As the title, we add decorator "dygraph_to_static_code", and add related "get_code", "get_func", "get_output" for ProgramTranslator. Next step will be adding "dygraph_to_static_program"

test=develop