[Executorch][llm] Make custom update cache op operate on indices (#10834)

This PR was created by the merge bot to help merge the original PR into
the main branch.
ghstack PR number: https://github.com/pytorch/executorch/pull/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
@diff-train-skip-merge

Co-authored-by: Kimish Patel <kimishpatel@fb.com>