[ExecuTorch][llm] Add source-transform and Meta-kernel tests for MoE op (#21536)

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/21122 by
@digantdesai
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and reviews
ghstack PR base:
https://github.com/pytorch/executorch/tree/gh/digantdesai/72/base
ghstack PR head:
https://github.com/pytorch/executorch/tree/gh/digantdesai/72/head
Merge bot PR base:
https://github.com/pytorch/executorch/tree/gh/digantdesai/71/orig
Merge bot PR head:
https://github.com/pytorch/executorch/tree/gh/digantdesai/72/orig
Differential Revision:
[D102381997](https://our.internmc.facebook.com/intern/diff/D102381997/)
@diff-train-skip-merge

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