Created by: liym27

1. modify astor.to_source(gast_node) to astor.to_source(gast.gast_to_ast(ast_node)), because astor.to_source() can't transform gast_node

2. Remove some dygraph APIs which don't have to be transformed eg: Conv2D, Linear

3. Remove repetitive functions in file utils.py and static_analysis.py