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
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* fix unittest
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* support reset param with None value

* add unittest
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* update unittest
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test=develop

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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.

test=develop

* fix circular dependent

* try import layers.nn from dygraph
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* allow sublayer or param shadow attrs

* add unittest
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* change remove fn name
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* DataParallel state dict don't include _layers.; test=develop

* add unitest of data parallel; test=develop

* add load state test; test=develop

* fix save_dygraph with nonexistent dir
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* minor fix
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* fix unittest
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* fix static save
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* use snake_cased Layer name

* add named_parameters and named_sublayers api

* add include_sublayers param, add unittest
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* fix named unittests
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* fix unittest
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* add api docs
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* arg fix
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* reserve rnn_impl name_scope for static graph
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* fix load static param
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* fix load static param
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* add clear_gradients for Optimizer, add api description, test=develop

* fix optest for optimizer's clear_gradient interface, test=develop

* add sample code, test=develop

* polish sample code, test=develop

* update layers, test=develop

* update layers for resnet, test=develop

* fix is_test attr, test=develop

* update cycle_gan, test=develop

* update reinforcement_learning, test=develop

* update ocr, test=develop

* fix bug, test=develop

* update PRelu arg
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* fix unittests
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* fix element mode alpha shape
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* split channel_or_input_shape arg
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* fix unittest
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* modify fc to linear in sample code, test=develop

* remove FC, test=develop

* remove warnings, test=develop

* drop fluid/imperative/README.md , test=develop

* change fc to linear, test=develop

* polish code style, test=develop

test=develop

* support dygraph structured name; test=develop

* add load static param unitest and fix save load; test=develop

* fix varBase import error; test=develop

* fix unitest error; test=develop

* add comment for parameter; test=develop

* fix uni test error; test=develop

* change parallel se-resnet; test=develop

* fix dygraph se resnext parallel test; test=develop

* remove useless code; test=develop

* remove useless code; test=develop

* add dygraph LayerList

* add unittest
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* add newline
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* typo fix
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* move private weight properties to public ones
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* revert changes to FC
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* fix unittest
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* fix unittest
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* fix coverage
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* fix merged dev
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* bug fix
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* expost fluid.dygraph.TracedLayer apis, test=develop

* polish doc, test=develop

* follow comments, test=develop, test=document_fix

* follow comments, test=develop

* remove save_inference_model return value, test=develop

* update layers used in mnist dygraph model, test=develop

* fix import issue, test=develop

* add dygraph utils, test=develop

* add unittest, test=develop

* remove params in Tracer object, test=develop

* Repair failed optest, test=develop

* remove build_once & name_scope (Conv2D)
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* fix unittest
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* Conv2DTranspose

* Conv3D & Conv3DTranspose
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* Pool2D & BatchNorm

* Embedding

* LayerNorm

* GRUUnit & NCE

* PRelu

* BilinearTensorProduct

* GroupNorm & SpectralNorm

* TreeConv
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* fix LayerNorm in transformer unnittest
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* disable LayerNorm or BatchNorm in multicard
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* refine Layer.create_parameter api
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* refine LayerNorm, remove begin_norm_axis param, add normed shape check
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* LayerNorm bug fix
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* fix optest,test=develop

* fix optest, test=develop

* fix optest for pass parameter_list when constructing an Optimizer class instance, test=develop

* polish code for better code style, test=develop

* fix se_resnext optest, test=develop

* polish code for better code style, test=develop
Co-authored-by: Nsongyouwei <youwei0314@gmail.com>

* add ParameterList container

* add unittest
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* fix container, rebase remove build_once
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* add parameters None check
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* update Sequential unittest
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* use Linear in Sequential sample code
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* fix Sequential arg type doc
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* update layers, test=develop

* fix input numpy, test=develop

* fix bugs, test=develop

* follow commments, test=develop

* update getitem, test=develop

test=develop

* add Linear layer
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* update unittest for coverage
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adds unary operator __neg__ for VarBase in dygraph mode, and for Variable in static graph mode.

* remove build_once & name_scope (Conv2D)
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* fix unittest
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* Conv2DTranspose

* Conv3D & Conv3DTranspose
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* Pool2D & BatchNorm

* Embedding

* LayerNorm

* GRUUnit & NCE

* PRelu

* BilinearTensorProduct

* GroupNorm & SpectralNorm

* TreeConv
test=develop

* fix LayerNorm in transformer unnittest
test=develop

* disable LayerNorm or BatchNorm in multicard
test=develop

* refine Layer.create_parameter api
test=develop

* refine LayerNorm, remove begin_norm_axis param, add normed shape check
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* LayerNorm bug fix
test=develop