[Executorch][llm] Make custom update cache op operate on indices

[pytorchbot]

This PR was created by the merge bot to help merge the original PR into the main branch.
ghstack PR number: #10610 by @kimishpatel
^ Please use this as the source of truth for the PR details, comments, and reviews
ghstack PR base: https://github.com/pytorch/executorch/tree/gh/kimishpatel/187/base
ghstack PR head: https://github.com/pytorch/executorch/tree/gh/kimishpatel/187/head
Merge bot PR base: https://github.com/pytorch/executorch/tree/gh/kimishpatel/186/orig
Merge bot PR head: https://github.com/pytorch/executorch/tree/gh/kimishpatel/187/orig
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• [PREEMPTIVE] Removal of ephemeral variants on scale-config.yml

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