* add BaseNode/BaseGraph, ModuleNode/ModuleGraph
* add docstring, redesign some functions
* add 'placeholder' after cat/bind/pass
* change type to a property from a method
* add test model s
* rename XXXNode in path to PathXXXNode
* 'xxconv' -> 'xxconv2d' in type
* ToGraph -> GraphConverter, PathToGraph -> PathToGraphConverter
* rm init_from_path_list
* convert some public methods to private methods in GraphConverter
* type -> basic_type
* fix some error
Co-authored-by: liukai <liukai@pjlab.org.cn>
LKJacky