export and delegate example fix, typos (#478)

Summary:
Pull Request resolved: https://github.com/pytorch/executorch/pull/478

- Fix error creating [composite and whole models](
https://www.internalfb.com/intern/staticdocs/executorch/docs/tutorials/setting_up_executorch/#step-4-generate-a-program-file-from-an-nnmodule)

- Add test to CI when model is add_mul

- Found some typos as well

Reviewed By: mergennachin

Differential Revision: D49602228

fbshipit-source-id: 285c7f59839ea5e0e04f662305e24d6ca98da518