Work around spec violation in executorch program unit test. (#453)

Summary:
Pull Request resolved: https://github.com/pytorch/executorch/pull/453

In theory, we shouldn't serialize an executorch program with default serializer because there is a spec violation with out variant graph.

Reviewed By: angelayi

Differential Revision: D49548576

fbshipit-source-id: cedf9810c594b33019eb50514e645c36289ce192