Update base for Update on "[Executorch][Export][2/N] Add getter utils to help with getting the artifacts easily"

Add getter utility methods to ExportSession for easier artifact retrieval.

- `get_exported_program` - to get output from export stage
- `get_edge_program_manager` - multiple stages produce edge program manager, for example, to_edge, transform, to_backend, go in reverse order to get the most processed edge program manager.

Differential Revision: [D87576720](https://our.internmc.facebook.com/intern/diff/D87576720/)

[ghstack-poisoned]

Author: abhinaykukkadapu