[export] Fix autogenerated stacktrace (#108217)
Summary: Existing code is incorrectly overwriting the stacktrace to be None because since there is no exception happening, `traceback.format_exc` is None. Also we should only populate the stack trace if it not there in the first place.
Test Plan: CI
Differential Revision: D48818478
Pull Request resolved: https://github.com/pytorch/pytorch/pull/108217
Approved by: https://github.com/zhxchen17
