restucture debug handle

Pull Request resolved: https://github.com/pytorch/executorch/pull/7197

This diff formats the debug handle generation process in et stack by. extracting bfs graph tracing process.


ghstack-source-id: 257880795
@exported-using-ghexport

Differential Revision: [D66622890](https://our.internmc.facebook.com/intern/diff/D66622890/)

Co-authored-by: gasoonjia <gasoonjia@icloud.com>