Add grammar fields to GenerationConfig for constrained decoding

[kirklandsign]

Summary:
Add grammar (string) and grammar_type (string) fields to GenerationConfig.
Empty-string defaults, no behavior change when empty. Prepares for
grammar-constrained decoding integration (e.g. JSON schema, regex).

Differential Revision: D105277261

[pytorch-bot]

🔗 Helpful Links

🧪 See artifacts and rendered test results at hud.pytorch.org/pr/pytorch/executorch/19616

• 📄 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:

• Run pull request jobs on OSDC runners in shadow mode

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

[meta-codesync]

@kirklandsign has exported this pull request. If you are a Meta employee, you can view the originating Diff in D105277261.

[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.

[Copilot]

Pull request overview

Adds grammar-related fields to the LLM runner GenerationConfig as groundwork for future constrained decoding support.

Changes:

• Adds grammar for constrained decoding definitions.
• Adds grammar_type to identify the grammar format.

---

💡 Add Copilot custom instructions for smarter, more guided reviews. Learn how to get started.