Polish UI (#5319)

Summary:
Enable all iOS 18 features by a single `--coreml-ios 18` argument, rather than one arg for each individual feature

Export our current best model by
```
python -m examples.models.llama2.export_llama \
    -c <download-path>/consolidated.00.pth \
    -p <download-path>/params.json \
    --disable_dynamic_shape -kv \
    --coreml \
    --coreml-ios 18 \
    --coreml-quantize b4w
```
i.e.
* FP16 activation
* Static sequence length
* In-place KV cache
* Fused scaled dot product attention
* 4-bit per-block weight

Pull Request resolved: https://github.com/pytorch/executorch/pull/5319

Reviewed By: kirklandsign

Differential Revision: D62616303

Pulled By: cccclai

fbshipit-source-id: 416a8acec785662ad76d04ea0b949ce3393308fe