Create an initital recipe for xnnpack optimization (#145)

Summary:
Pull Request resolved: https://github.com/pytorch/executorch/pull/145

This is just a very initial attempt to define composable recipes for our demos.

**The goal for this diff is just to move xnnpack specific stuff out from the `examples/models` to `examples/recipes/xnnpack_optimization`**.

In follow-up diffs, we will continue moving pieces from `examples/backend` to create a complete recipe to demonstrate xnnpack optimization e2e.

Reviewed By: kirklandsign

Differential Revision: D48704007

fbshipit-source-id: ccda52a4a7b742a8dec16dc92413803f5e8417bb