Demo CoreML Partitioner (#2384)

Summary:
We had a demo in [PR 1981](https://github.com/pytorch/executorch/pull/1981) by adding a unit test. This time, let us give an e2e demo: partition real example models and serialize to `.pte`

2 changes:
1. Rename `examples.apple.coreml.scripts.export_and_delegate` to `examples.apple.coreml.scripts.export`
2. Add `--use_partitioner` command line argument to use partitioner

Pull Request resolved: https://github.com/pytorch/executorch/pull/2384

Reviewed By: kirklandsign

Differential Revision: D54840049

Pulled By: shoumikhin

fbshipit-source-id: 8c4b16b206666ea3c7b8863b7fc25cb7a903b836