[export] Add aot_export 1/N (#101490)
This PR adds aot_export_module as the lowering path from torch.level graph to aten graph. Some known limitations that need to be addressed in the follow up PRs:
1. Store param/buffer data in ExportedProgram
2. Fully support torch.cond with params/buffers
3. Making sure no duplicated ExportMetaData entry
4. This API will break Executorch if used on PyE, we will figure out a plan internally.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/101490
Approved by: https://github.com/avikchaudhuri
