support skip atten in export

[jackzhxng]

Summary:
Support export for llama model variants with attention layer skipping. We only need to specify the attention skip patterns in config.json in layer_type. E.g.,

"layer_types": [
"full_attention",
"full_attention",
"full_attention",
"skip_attention",
"skip_attention",
"skip_attention"
]

Differential Revision: D88399533

[pytorch-bot]

🔗 Helpful Links

🧪 See artifacts and rendered test results at hud.pytorch.org/pr/pytorch/executorch/16104

• 📄 Preview Python docs built from this PR

Note: Links to docs will display an error until the docs builds have been completed.

❗ 1 Active SEVs

There are 1 currently active SEVs. If your PR is affected, please view them below:

• Replace all macOS instances with nextjs due CVE-2025-55182

✅ You can merge normally! (1 Unrelated Failure)

As of commit 72fd468 with merge base 4014597 ():

BROKEN TRUNK - The following job failed but were present on the merge base:

👉 Rebase onto the `viable/strict` branch to avoid these failures

• pull / android / run-emulator (gh) (trunk failure)
  Timeout waiting for emulator to boot.

This comment was automatically generated by Dr. CI and updates every 15 minutes.

[meta-codesync]

@jackzhxng has exported this pull request. If you are a Meta employee, you can view the originating Diff in D88399533.

[github-actions]

This PR needs a release notes: label

If your change should be included in the release notes (i.e. would users of this library care about this change?), please use a label starting with release notes:. This helps us keep track and include your important work in the next release notes.

To add a label, you can comment to pytorchbot, for example
@pytorchbot label "release notes: none"

For more information, see
https://github.com/pytorch/pytorch/wiki/PyTorch-AutoLabel-Bot#why-categorize-for-release-notes-and-how-does-it-work.