[ExecuTorch][llm] Add MoE quantized custom op documentation

[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 MoE section to README: supported configs, AOT export, runtime build,
validation instructions.

Differential Revision: D102382001

[pytorch-bot]

🔗 Helpful Links

🧪 See artifacts and rendered test results at hud.pytorch.org/pr/pytorch/executorch/21125

• 📄 Preview Python docs built from this PR

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❌ 1 New Failure, 1 Unrelated Failure

As of commit 49ad534 with merge base bfed808 ():

NEW FAILURE - The following job has failed:

• pull / unittest-buck / linux / linux-job (gh)
  RuntimeError: Command docker exec -t dea9d89d7c8cc9672ddfd7eec5bbbb5a22c5447528153dd406dcdeb1ccce3132 /exec failed with exit code 3

BROKEN TRUNK - The following job failed but were present on the merge base:

👉 Rebase onto the `viable/strict` branch to avoid these failures

• pull / unittest-buck / macos / macos-job (gh) (trunk failure)
  RuntimeError: Command bash /Users/ec2-user/runner/_work/_temp/exec_script failed with exit code 3

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