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

* 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

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