Qwen 3.5 MoE Metal: Use max-sized prefill example for dynamic inputs

[manuelcandales]

With alloc_graph_input=False, ExecuTorch sets the input tensor's
numel_bound_ from the serialized example size. A small example (T=2)
prevents runtime inputs larger than 2 tokens. Use max_seq_len-1 as
the prefill example size so any prompt length is accepted at runtime.

Authored with Claude.

[manuelcandales]

Stack from ghstack (oldest at bottom):

• -> Qwen 3.5 MoE Metal: Use max-sized prefill example for dynamic inputs #18956
• 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]

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🧪 See artifacts and rendered test results at hud.pytorch.org/pr/pytorch/executorch/18956

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• [CI[B200] Smoke test encounters CUDA Unknown error for dgxb200-03 and dgxb200-04

❌ 1 New Failure, 1 Cancelled Job, 4 Unrelated Failures

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• pull / unittest-editable / macos / macos-job (gh)
  export/tests/test_target_recipes.py::TestTargetRecipes::test_mv3_model

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• pull / unittest / windows / windows-job (gh)

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• pull / test-models-linux (resnet50, portable, linux.2xlarge) / linux-job (gh) (detected as infra flaky with no log or failing log classifier)
• pull / test-samsung-quantmodels-linux / linux-job (gh) (detected as infra flaky with no log or failing log classifier)

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

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

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