Support Various-Length Return Grammar in Dy2stat. This PR is a follow-up of https://github.com/PaddlePaddle/Paddle/pull/25176 .

The basic idea is putting no-value placeholder variables at `return` statement to make all `return` statement have same length, after that the static graph can have fixed fetch output (code at return_transformer.py). Then remove those no-value placeholder when we finally return dygraph result (code at partial_program.py).

However, various length return in Bert model is still not supported. The dy2stat can change the code as I wish but some ops which check shape at compile time (e.g. Reshape, MatMul) will throw error because of the no-value-placeholder may not have the required shape. Is this a matter? To me, those no-value placeholder will be replaced as really values meeting shape requirements at run time, so I think the solution should be some way to do the compile-time checking. By the way, every time when we have dynamic shape, it often causes problem in dy2stat. We should find a way to handle it in the future.

Fixing various return in Bert is my TODO thing and I will also find some other existing models for verification.

* support to save varBase created in __init__ test=develop

* polish code test=develop

* refine to_static_var test=develop

* refine warning test=develop

* add unitteset for to_static_var test=develop

* fix logger test=develop

* polish buffers en doc test=develop

* fix param_guard test=develop

* refine en doc test=develop

This PR added basic support for 'return' grammar in dy2stat. It supports the control flow of 'return'.

The basics idea is using a return value variable to store the early return statements and boolean state variables with if-else to skip the statements after the return statements.

**This PR is very basic support. There are some corner cases I didn't develop/test**. For example, 'return None', 'return different length of variables', 'return non-tensor and tensor together', 'no return statement'. **These corner cases will be done in my next PRs**. Target date is this week.

**Note**: 
1. for the unit test, I changed test_program_translator.py because the StaticCode of `dyfunc_with_if_else` will change. To guarantee the correctness of `dyfunc_with_if_else`, I also run it in `TestRecursiveReturn` in test_return.py.

2. I commented the early return code in bert_dygraph_model.py because 'return different length of variables' is unsupported now. I also know that there are some other models used early return and we didn't enable it in the unit test. I will add support for it in next PRs and then re-enable those tests.

* add cycle_gan_model

* align train test=develop

* modify image_size into 64 to avoid TimeOut test=develop

* TODO in GPU test=develop

* Refine caches of converting func test=develop

* fix if statement test=develop

* refine cache code test=develop

* rm unuse import statement test=develop

* Polish code comment test=develop

* Refine check_type for @declarative test=develop

* The arg of append() can be not Tensor temporarily.

* Add Seq2Seq as ProgramTranslator Unit Test. 

* set dtype of vocab_size_tensor to int64 to pass Windows-CI. 

[Dy2Static]Convert var.shape stmt and Convert the return variables of Tensor-dependent 'if' staments to Tensor if it not  (#24911)

* Support int and long: int or long -> six.integer_types. 

* Modify test_tensor_shape: fix bug and modify comment. 

* Support convert_var_shape to convert var.shape stmt

* Modify code in ifelse_simple_func.py because don't support return non-Tensor in Tensor-dependent 'if' stament currently. 

* Convert the return variables of Tensor-dependent 'if' staments to Tensor if it not. test=develop

* Move function 'convert_len' to file convert_operators.py 

* Support that for statements are transformed to while statements.

* Fix bug: raise None -> return None. 

* Support variable loaded and created in loop.

* Use int64 in Py2 and Py3 in function to_static_variable.

* cast var in convert_logical_XX. 

* Add convert_ifelse function in convert_operators.py  

* Add logical_transformer. Remove LogicalTransformer from loop_transformer.py 

* Revert modified tests in PR24799(convert_while_stmt). 

* Comment and modify code that doesn't support `return` statement. 

* Remove unnecessary class: MergeAssignTransformer, NodeTestTransformer and IfConditionVisitor in ifelse_transformer. 

* support for iter & enumerate varbase, test=develop

* revert IsControlFlowVisitor change, test=develop

* Support return variable in only one of if body or else. 

* remove after_visit in IfElseTransformer.

* Modify the result of get_name_ids in test_ifelse_basic.py 

* Add unittest to test the new case. 

* Modify code according to reviews. 

* Support convert_while_loop. 

* Comment code that not supported 'if' in test_break_continue. 

* Convert int into tensor to support 'if' stmt in for/while loop. 

* Add unittest to test all cases of convert_logical_XX. 

* Add unittest to test all cases of convert_while_loop. 

* Fix bug in LogicalOpTransformer. test=develop

* support nested input and output test=develop

* remove code of convert type(output) in unittest test=develop

* add warning test=develop

* [dy2stat] Support len

* add convert_call func

* refine code test=develop

[Dy2Stat]Support to transform sequence assignments and multi-target assignments to normal assignments (#24643)

* fix bug with `if Tensor` in is_control_flow test=develop

* remove continue test=develop

* Support lambda and add unittest test=develop

* Support to create LoDTensorArray in control flow (cond and while_loop)

* Fix bug: return LoDTensorArray in while_loop

* Change code in list_transformer.py to accommodate the new features. 

* initial test

* for enumerate basic implement, test=develop

* update unittests, test=develop

* refine unittests to adapt new training mode, test=develop

* refactor for node stmts parsing code, test=develop

* self-review & polish details, test=develop

As the title.

* fix some print transformer problems, test=develop

* simplify writing & avoid bud, test=develop

* polish detail, test=develop

* fix convert call globals_funcs test=develop

* add import statement test=develop

Add assert grammar for ProgramTranslator

* fix bug with `if Tensor` in is_control_flow test=develop

* remove continue test=develop

* support save_inference_model in program_translator test=develop

* fix compatibility with OrderedDict.values() in python3 test=develop

* synchronized random_seed test=develop

* Polish Error Message test=develop

* [Dy2Stat] Add test for bert pretraining. 

* Construct fake data. 

* Synchronous random seed of program. 

* 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.

* 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

* 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.

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.