[Executorch][llm] Enable local global attention in export_llama script (#10836)

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/10612 by
@kimishpatel
^ Please use this as the source of truth for the PR details, comments,
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ghstack PR base:
https://github.com/pytorch/executorch/tree/gh/kimishpatel/189/base
ghstack PR head:
https://github.com/pytorch/executorch/tree/gh/kimishpatel/189/head
Merge bot PR base:
https://github.com/pytorch/executorch/tree/gh/kimishpatel/188/orig
Merge bot PR head:
https://github.com/pytorch/executorch/tree/gh/kimishpatel/189/orig
@diff-train-skip-merge

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