Add test for stateful model and fix output backings issue (#5294)

Summary:
Fixes a couple of issues
-  Prewarms stateful model
- The prediction was run twice if output backings was used.
- Adds unit test for stateful model

Pull Request resolved: https://github.com/pytorch/executorch/pull/5294

Reviewed By: kirklandsign

Differential Revision: D62551117

Pulled By: cccclai

fbshipit-source-id: 48835c3e841914ac11c1ea7a9246c548f2b16035