Add topk Triton kernel for CUDA backend

[mergennachin]

Add a Triton-based topk kernel that replaces aten.topk during graph
transformation, compiled directly into the AOTInductor .so via
wrap_triton (no C++ fallback shim needed).

The kernel uses iterative argmax with masking, adapted from
FlagGems/aiter. It is registered via @triton_op("triton::topk") and
auto-substituted for aten.topk.default through ReplaceEdgeOpWithTritonOpPass.

Tests follow the chunk_gated_delta_rule pattern: eager correctness
across 8 configs, export validation, and E2E C++ runner comparison.

[pytorch-bot]

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❌ 2 New Failures, 8 Unrelated Failures

As of commit b29e89e with merge base cc27e6b ():

NEW FAILURES - The following jobs have failed:

• Build Presets / windows (pybind) / build (gh)
  The process 'C:\Program Files\Git\cmd\git.exe' failed with exit code 1
• Build Presets / windows (windows) / build (gh)
  The process 'C:\Program Files\Git\cmd\git.exe' failed with exit code 1

FLAKY - The following jobs failed but were likely due to flakiness present on trunk:

• pull / unittest-editable / windows / windows-job (gh) (similar failure)
  The process 'C:\Program Files\Git\cmd\git.exe' failed with exit code 1
• Test CUDA Windows Export and E2E / test-model-cuda-windows-e2e (nvidia, parakeet-tdt, non-quantized) / windows-job (gh) (similar failure)
  The process 'C:\Program Files\Git\cmd\git.exe' failed with exit code 1

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

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

• pull / unittest / windows / windows-job (gh) (trunk failure)
• Test CUDA Windows Export and E2E / test-model-cuda-windows-e2e (mistralai, Voxtral-Mini-3B-2507, non-quantized) / windows-job (gh) (trunk failure)
  The process 'C:\Program Files\Git\cmd\git.exe' failed with exit code 1
• Test CUDA Windows Export and E2E / test-model-cuda-windows-e2e (mistralai, Voxtral-Mini-3B-2507, quantized-int4-weight-only) / windows-job (gh) (trunk failure)
  The process 'C:\Program Files\Git\cmd\git.exe' failed with exit code 1
• Test CUDA Windows Export and E2E / test-model-cuda-windows-e2e (mistralai, Voxtral-Mini-4B-Realtime-2602, quantized-int4-tile-packed) / windows-job (gh) (trunk failure)
  The process 'C:\Program Files\Git\cmd\git.exe' failed with exit code 1
• Test CUDA Windows Export and E2E / test-model-cuda-windows-e2e (nvidia, diar_streaming_sortformer_4spk-v2, non-quantized) / windows-job (gh) (trunk failure)
  The process 'C:\Program Files\Git\cmd\git.exe' failed with exit code 1
• Test CUDA Windows Export and E2E / test-model-cuda-windows-e2e (nvidia, parakeet-tdt, quantized-int4-weight-only) / windows-job (gh) (trunk failure)
  The process 'C:\Program Files\Git\cmd\git.exe' failed with exit code 1

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[Copilot]

Pull request overview

Adds a Triton-based topk kernel for the ExecuTorch CUDA backend, replacing aten.topk.default during graph transformation. The kernel uses iterative argmax/argmin with masking and is registered via @triton_op.

Changes:

• New Triton topk kernel implementation with iterative max/min and masking algorithm
• Registration of the kernel in the edge-to-triton replacement pass
• Tests (eager correctness, export validation, E2E C++ runner) and a dedicated C++ test runner

Reviewed changes

Copilot reviewed 6 out of 6 changed files in this pull request and generated no comments.

Show a summary per file

File	Description
backends/cuda/triton/kernels/topk.py	New Triton topk kernel and its abstract/fake implementation
backends/cuda/triton/kernels/init.py	Export the new topk symbol
backends/cuda/triton/replacement_pass.py	Map aten.topk.default to the Triton kernel
backends/cuda/tests/test_topk.py	Eager correctness, export, and E2E tests
backends/cuda/tests/topk_runner/main.cpp	C++ runner for E2E testing
backends/cuda/tests/topk_runner/CMakeLists.txt	Build config for the C++ runner

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