Metal backend: Add topk fallback kernel via MPSGraph

[manuelcandales]

Adds aoti_torch_mps_topk using MPSGraph's topKWithSourceTensor. Required
for MoE expert routing (torch.topk in SparseMoE.forward). Supports
arbitrary dim via transpose-topk-transpose, largest/smallest modes,
float32 and bfloat16. Includes MPSGraph caching and int32-to-int64
indices conversion (AOTInductor expects int64, MPSGraph outputs int32).

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]

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❌ 1 New Failure, 3 Unrelated Failures

As of commit 1fbb94f with merge base 5707e2a ():

NEW FAILURE - The following job has failed:

• Test CoreML Backend / test-coreml / test-backend-macos (coreml, models) / macos-job (gh)
  RuntimeError: Command bash /Users/ec2-user/runner/_work/_temp/exec_script failed with exit code 1

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

• pull / test-binary-size-linux-gcc / 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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