[ExecuTorch][llm] Add source-transform and Meta-kernel tests for MoE op

[digantdesai]

Stack from ghstack (oldest at bottom):

• [ExecuTorch][llm] Wire --use_moe_quantized_op through export pipeline and LLM config #21126
• [ExecuTorch][llm] Add MoE quantized custom op documentation #21125
• [ExecuTorch][llm] Fuse w1+w3 into single GEMM in quantized_moe_ffn #21124
• [ExecuTorch][llm] Add numerical kernel tests for quantized_moe_ffn #21123
• -> [ExecuTorch][llm] Add source-transform and Meta-kernel tests for MoE op #21122
• [ExecuTorch][llm] Add MoE source transformation with INT4 quantized packing #21121
• [ExecuTorch][llm] Add Meta kernel and registration probe for quantized_moe_ffn #21120
• [ExecuTorch][llm] Add quantized_moe_ffn custom op with C++ kernel #21119

Add test_quantized_moe.py coverage for source transformation, INT4 packing, Meta-kernel validation, kernel registration, and shared-expert behavior.

Exercise the production torchao gate quantization recipe and assert that the transformed module exports a llama.quantized_moe_ffn call with an fp32 gate buffer.

Differential Revision: D102381997

[pytorch-bot]

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