Fix xnnpack demo (#122)

Summary:
Pull Request resolved: https://github.com/pytorch/executorch/pull/122

Fixed several issues with the XNNPACK demo:
  - broken buck build in OSS (see errors in P813564245)
     - `buck2 build //examples/backend:xnn_executor_runner` doesn't build
     - `buck2 build //examples/backend:xnnpack_examples` doesn't build
  - broken `README` file, e.g. missing `.md` so no format at all ([see the file in OSS](https://github.com/pytorch/executorch/blob/main/examples/backend/README))
  - broken generated file names, e.g. ugly name like `mv2_xnnpack_.pte` or `mv2_xnnpack__quantize.pte`

Reviewed By: huydhn, kirklandsign

Differential Revision: D48661299

fbshipit-source-id: d9fd89f6ffe565023c0f9a9a9ccf1415a3c59024