[ExecuTorch][llm] Add quantized_moe_ffn custom op with C++ kernel (#21533)

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/21119 by
@digantdesai
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and reviews
ghstack PR base:
https://github.com/pytorch/executorch/tree/gh/digantdesai/69/base
ghstack PR head:
https://github.com/pytorch/executorch/tree/gh/digantdesai/69/head
Merge bot PR base: https://github.com/pytorch/executorch/tree/main
Merge bot PR head:
https://github.com/pytorch/executorch/tree/gh/digantdesai/69/orig
Differential Revision:
[D102382000](https://our.internmc.facebook.com/intern/diff/D102382000/)
@diff-train-skip-merge

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