* add double grad implementation for dygraph, test=develop

* polish code, add uts, test=develop

* fix place bug, test=develop

* polish codes, add more uts for coverages, test=develop

* add no_grad_set, test=develop

* add star gan ut, test=develop

* follow comments, test=develop

As the title

* move __getitem__ to cpp

* bug fix

* add type check and gil release

* support negative step with omitted ends
test=develop

* code refine
test=develop

* bug fix
test=develop

* slice always return different pyobj
test=develop

* Support to use external function test=develop

* refine the parms of ast_to_func test=develop

1. Add support for Break and Continue in Dygraph to Static
2. Also add support for gast.Not in NodeTestTransformer
3. Also add support for logical op transformation in LoopTransformer

* Support and/or in controlFlow if test=develop

* Support Tensor.shape in control flow if/for/while and separate TensorShapeTransformer from BasicApiTransformer. test=develop

* Support and/or in controlFlow if test=develop

* Refine IsControlFlow interface test=develop

* add dygraph parallel env doc, test=develop

* polish details, test=develop, test=document_fix

* fix examples error in other apis, test=develop

* fix more example error in other api, test=develop

* add white list for gpu examples, test=develop, test=document_fix

* support list in control flow if. test=develop

* support list in for/while and supplement tests. test=develop

1. Add basic support for `for in range` loop
2. Move `test_dygraph_to_static_*` to `dygraph_to_static` dir and rename them
3. Add test case for dict in while_loop

+ Refine code structure and move code related with control flow `if` into `ifelse_transformer.py`
+ Merge code of `ast_utils.py`  into `utils.py` 

* refine grad maker, test=develop

* refactor tracer stage 1, test=develop

* merge develop to solve conflict third times, test=develop

* support basic tensor.shape. 

* Support tensor.shape with dependencies. 

1. copy.deepcopy in NameVisitor should be changed to copy.copy to make hash or set work
2. read_context should be type of gast.Load()/gast.AugLoad(), not gast.Load/gast.AugLoad

Add basic support for while in translating dygraph to static

1. Analysis the variable liveness in class NameVisitor
2. Replace while key word using while_loop API

* Modify the way of inserting newly defined func_nodes test=develop

* functional dygraph enable/disable
test=develop

* use context manager instead
test=develop

* refine sample code
test=develop

* rename api & expose to fluid
test=develop

* fix sample code
test=develop

* change dygraph_to_static_output to dygraph_to_static_graph. test=develop

* Remove duplicate code. test=develop

* Follow comments from Liujie. test=develop

* change dygraph_to_static_output to dygraph_to_static_graph. test=develop

* Support fetch and run program in the process of dygraph_to_static_output. test=develop

* fix to_source(gast) and remove dygraph API such as Conv2D, Linear. test=develop

1. Considering functions, I have to change the node type from single value to a set. Because python function is allowed to return different types. The set represent all possible types

2. I added scope_name and scope_type for AstVarScope, because in python functions, variable may have different scope. For example:

```
a = 3
def foo(b):
    a = 9
    return a + b
```

the `a` in `foo` is different to the `a` out of `foo`. Similar to class field. The scope_name will help me to know the function name when static analysis finds a `return` sentence.

* add is_control_flow_if  test=develop

* support dygraph to static graph for simple case.

* add test for dygraph API recognition. test=develop

* support basic to_variable api. test=develop

* update dict: dygraph_class_to_static_api

* add all tests of dygraph api. test=develop

* use gast/astor instead of ast/codegen for the compatibility of PY2 and PY3. test=develop

* add arg 'num_flatten_dims' for fc ast node. test=develop

* Modify names of class by Camel-Case.

* support nested if/else

* support to derivate returns the parameter list automatically

* polish tranform function of slice

* fix modify x.numpy()[i] slice function

* support to transform ast.node into callable function

* fix get_name_ids bug and add more unittest test=develop

* fix requirements.txt test=develop

* remove useless import statement test=develop

* Fixed version compatibility issues in param of function test=develop

* use decorater to test ast_to_func test=develop

* add textwrap.dedent for source_code test=develop

* polish code comment

* fix compatibility with python2 and python3 test=develop

* fix gast version error test=develop

* fix gast repo test=develop

* polish transfer_from_node_type code test=develop

* add nested_if_else unittest test=develop

* split IfElseTransformer test=develop

* specify gast version test=develop

* fix ast_to_func root type test=develop

1. Move AstNodeWrapper, StaticAnalysisVisitor to a new python file: static_analysis.py
2. Add basic node var type analysis

* Linear use mul op
test=develop

* fix unittest
test=develop

* support reset param with None value

* add unittest
test=develop

* update unittest
test=develop

test=develop

test=develop

As the title

This PR provides very basic and simple framework for transforming Dygraph to Static Graph.

API names, final outputs are not determined yet. Feel free to modify or add class/function/type when you think the framework is not extendable for you.