[export] Don't error if nn_module_stack doesn't contain a class (#119753)
Summary: When we deserialize nn_module_stack, sometimes the module no longer exists in the python environment so we cannot deserialize it back into the python type and instead it's kept as a string. This causes downstream failures when retracing due to one of our checks in export. This diff just bypasses the check.
Test Plan: CI
Reviewed By: chakriu
Differential Revision: D53527706
Pull Request resolved: https://github.com/pytorch/pytorch/pull/119753
Approved by: https://github.com/zhxchen17
