[export] skip nn_module_stack verifier for non-fx.GraphModule modules (#122210)
Downstream users of torch.export may have different module classes (e.g. LoweredBackendModule), which cannot be checked for metadata in the same way. Add lines to skip this for non-fx.GraphModule modules.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/122210
Approved by: https://github.com/angelayi, https://github.com/zhxchen17
