[ExecuTorch][llm] Add MoE source transformation with INT4 quantized packing (#21535)

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/21121 by
@digantdesai
^ 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/digantdesai/71/base
ghstack PR head:
https://github.com/pytorch/executorch/tree/gh/digantdesai/71/head
Merge bot PR base:
https://github.com/pytorch/executorch/tree/gh/digantdesai/70/orig
Merge bot PR head:
https://github.com/pytorch/executorch/tree/gh/digantdesai/71/orig
Differential Revision:
[D102381994](https://our.internmc.facebook.com/intern/diff/D102381994/)
@diff-train-skip-merge

Co-authored-by: Digant Desai <digantdesai@meta.com>