Qwen 3.5 MoE: Add --backend metal export path

[manuelcandales]

Adds Metal backend support to export.py via --backend metal flag:

• _prepare_and_quantize_metal: applies source transforms, quantizes
  experts to MLX affine INT4, quantizes non-expert layers with fpa4w
  (skips shared_expert_gate with N<4 for prefill compatibility)
• _export_metal: exports decode + prefill methods via MetalBackend/
  MetalPartitioner

CUDA and MLX paths are unchanged.

Authored with Claude.

[manuelcandales]

Stack from ghstack (oldest at bottom):

• Qwen 3.5 MoE: Add Metal build target #18881
• -> Qwen 3.5 MoE: Add --backend metal export path #18880
• Qwen 3.5 MoE: Add Metal source transformations #18879
• Metal backend: Add gated delta rule kernel for linear attention #18878
• Metal backend: Add gather_qmv kernel for MoE expert-indexed quantized matmul #18877
• Metal backend: Add topk fallback kernel via MPSGraph #18876
• Metal backend: Add SDPA head_dim=256 support #18875
• Metal backend: Add INT32 and BOOL dtype support #18874

[pytorch-bot]

🔗 Helpful Links

🧪 See artifacts and rendered test results at hud.pytorch.org/pr/pytorch/executorch/18880

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❌ 3 New Failures, 3 Unrelated Failures

As of commit d70d646 with merge base 5707e2a ():

NEW FAILURES - The following jobs have failed:

• Cadence Build & Test / cpu-test / test-aot / test-aot (gh)
  backends/cadence/aot/tests/test_replace_ops_passes.py::TestReplaceOpsPasses::test_replace_transposed_conv_with_linear_1
• pull / unittest-arm-backend-with-no-deps (test_pytest_ops_tosa) / linux-job (gh)
  RuntimeError: Command docker exec -t 9c0022e80bc1708dde5860640085cda36db631c5d322685747f97ce8a83535ca /exec failed with exit code 1
• pull / unittest-editable / macos / macos-job (gh)
  export/tests/test_target_recipes.py::TestTargetRecipes::test_mv3_model

FLAKY - The following job failed but was likely due to flakiness present on trunk:

• pull / test-vulkan-models-linux / linux-job (gh) (detected as infra flaky with no log or failing log classifier)

BROKEN TRUNK - The following jobs failed but was present on the merge base:

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

• pull / unittest / windows / windows-job (gh) (trunk failure)
  ##[error]The operation was canceled.
• pull / unittest-editable / windows / windows-job (gh) (trunk failure)
  ##[error]The operation was canceled.

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