Add const qualifier to multimodal prefill

[kirklandsign]

Summary

Easier for rvalue inputs

Test plan

CI

[pytorch-bot]

🔗 Helpful Links

🧪 See artifacts and rendered test results at hud.pytorch.org/pr/pytorch/executorch/14494

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✅ No Failures

As of commit 54ddc52 with merge base c9f46e2 ():
💚 Looks good so far! There are no failures yet. 💚

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[github-actions]

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[kirklandsign]

@pytorchbot cherry-pick --onto release/1.0 -c fixnewfeature

[pytorchbot]

Cherry picking #14494

The cherry pick PR is at #14523 and it is recommended to link a fixnewfeature cherry pick PR with an issue. The following tracker issues are updated:

• [v1.0.0] Release Tracker #14288 (comment)

Details for Dev Infra team

Raised by workflow job