* add cast transform and its UT for dygraph_to_static. 

* fix test_lac timeout test=develop

* disable it on windows test=develop

* add sentiment unittest

* add LAC model

* add test=develop

* rename tmp test=develop

* fix timeout test=develop

* undo tmp_var test=develop

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.

* fix GPU diff test=develop

* refine code test=develop

* 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

* add for iter var list, test=develop

* add enumerate unittest, 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. 

Some big models can timeout on Windows CPU machine. I added some timeout properties.

Add TSM as ProgramTranslator Unit Test. The TSM code is referred from PaddlePaddle/models#4229

* add MobileNet unittest test=develop

* fix cudnn random test=develop

[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

[Dy2stat] decrease the batch size to decrease GPU usage.

* 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.

[Dy2stat] Add word2vec as unittest

* 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. 

The random failure at Windows may due to some random gt_boxes can cause some numbers in YoloV3 to be negative thus access the invalid memory. This PR tries to solve it.

Add Yolov3 as ProgramTranslator Unit Test. The YoloV3 code is referred from PaddlePaddle/models/dygraph/yolov3

* add test_bmn_model test=develop

* remove random test=develop

* 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

* Revert "Fix test_bert on GPU (#24692)"

This reverts commit 62222bf4.

* fix random bug in bert unittest, test=develop

DataLoader makes the data diff even if the data of reader is the same on CUDA place. This PR doesn't use DataLoader to pass the test. we will use DataLoader back after we fix it.

[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

* test_seresnet support gpu test=develop

* Set the Flag by core.globals test=develop

* modify into set_flags test=develop