Support graphs which return get_attr nodes directly as output

Summary:
X-link: https://github.com/pytorch/pytorch/pull/107610

Currently serializing graphs which return get_attr's directly as output fails. This diff adds support for that only in EXIR serializer while we still support unlifted params.

Reviewed By: angelayi

Differential Revision: D48258552

fbshipit-source-id: 741e6166db86492fdff1ef0b1647601ee351ff77