[export] Improve consistency for nn_module_stack metadata, add checks to _trace.py (#120661)
We would like to improve consistency for nn_module_stack metadata in torch.export.
This PR ensures that all tests in test/export/test_export.py has the following constraints:
- Remove nn_module_stack for all placeholder & output nodes, for all modules and submodules
- Ensure nn_module_stack is present for all other node types for the top-level module (there is still an issue with torch.cond submodules having empty fields)
- Add these checks to _export() in _trace.py (we would add this in the Verifier, but downstream apps construct ExportedPrograms separate from _export(), and metadata may not be maintained there)
Pull Request resolved: https://github.com/pytorch/pytorch/pull/120661
Approved by: https://github.com/avikchaudhuri
