Refactor ExportedProgram to expose the functions for pre and postprocessing (#119513)
Reason:
Consumers of ExportProgram might choose to further lower exported_program.graph_module
to something else.
Then, it will need to setup the calling convention to call it.
This refactor concentrates these calling convention to one place and can be reused.
Fixes #ISSUE_NUMBER
Pull Request resolved: https://github.com/pytorch/pytorch/pull/119513
Approved by: https://github.com/zhxchen17
