Add CUDA argmax kernel for LLM sampler

[larryliu0820]

Add a CUDA kernel for argmax operation to support GPU-based sampling:

• argmax.cuh: Template kernel using warp-level reductions with __shfl_xor_sync
  for efficient parallel max finding. Supports float, half, and bfloat16.
• argmax.cu: Wrapper function argmax_cuda() that launches the kernel,
  handles device-to-host copy, and synchronization.
• test_argmax.cu: Comprehensive unit tests covering various vocab sizes,
  data types, edge cases, and numerical precision.
• CMakeLists.txt: Build configuration for extension_llm_sampler_cuda library
  and GoogleTest-based unit tests.

[larryliu0820]

Stack from ghstack (oldest at bottom):

• Integrate CUDA sampler into ASR runner and enable skip_copy for decoder #16388
• Add CudaSampler class for GPU-based token sampling #16387
• -> Add CUDA argmax kernel for LLM sampler #16386

[pytorch-bot]

🔗 Helpful Links

🧪 See artifacts and rendered test results at hud.pytorch.org/pr/pytorch/executorch/16386

• 📄 Preview Python docs built from this PR

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❌ 2 New Failures, 2 Unrelated Failures

As of commit f8cd4d2 with merge base c5d66a5 ():

NEW FAILURES - The following jobs have failed:

• Lint / lintrunner / linux-job (gh)
  >>> Lint for extension/llm/sampler/test/test_argmax.cu:
• pull / test-samsung-models-linux / linux-job (gh)
  test_inception_v3_fp16

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

• Test Metal Backend / export-model-metal-artifact (openai, whisper-small, non-quantized) / macos-job (gh) (matched macos rule in flaky-rules.json)
  File doesn't exist

UNSTABLE - The following job is marked as unstable, possibly due to flakiness on trunk:

• pull / android / run-emulator (gh) (#16137)
  Timeout waiting for emulator to boot.

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