Fix Windows CUDA build: guard extension_cuda compile options to CXX

[shoumikhin]

Problem

The Windows CUDA e2e build (test-model-cuda-windows-e2e) fails compiling the aoti_cuda_shims .cu kernels:

nvcc fatal : A single input file is required for a non-link phase when an outputfile is specified

The build fails -> --target install never finishes -> executorchConfig.cmake is missing -> every example-runner find_package(executorch) fails -> all 6 models go red.

Root cause

extension/cuda/CMakeLists.txt applied ${_common_compile_options} PUBLIC, unguarded (it expands to /wd4996 on MSVC). #20158 linked extension_cuda into slimtensor's INTERFACE, so the flag propagates extension_cuda -> slimtensor -> aoti_cuda_shims into the .cu nvcc compile as a bare /wd4996, which nvcc parses as a second input file. Every other target in the CUDA stack already guards these with $<COMPILE_LANGUAGE:CXX>; this one didn't.

Fix

Guard the compile options with $<COMPILE_LANGUAGE:CXX>, matching the rest of the stack.

[pytorch-bot]

🔗 Helpful Links

🧪 See artifacts and rendered test results at hud.pytorch.org/pr/pytorch/executorch/20184

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❌ 1 New Failure, 11 Pending, 4 Unrelated Failures

As of commit 0d1780c with merge base 4229704 ():

NEW FAILURE - The following job has failed:

• pull / test-multimodal-linux (gemma3-4b) / linux-job (gh)
  RuntimeError: Command docker exec -t f9ed58f2dc3d15513e92d70f9d9bae1a3511389d0e3568616b630d3868d04f28 /exec failed with exit code 139

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

• periodic / test-models-linux (cmake, mv3, xnnpack-quantization-delegation, linux.2xlarge, 90) / linux-job (gh) (detected as infra flaky with no log or failing log classifier)
• pull / test-models-linux (linear, portable, linux.2xlarge) / linux-job (gh) (matched linux rule in flaky-rules.json)
  The process '/usr/bin/git' failed with exit code 128
• pull / test-qnn-testsuite-linux / test-backend-linux (qnn, models) / linux-job (gh) (detected as infra flaky with no log or failing log classifier)
• pull / test-static-llama-qnn-linux (stories_110m) / linux-job (gh) (detected as infra flaky with no log or failing log classifier)

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[Copilot]

Pull request overview

This PR fixes a Windows CUDA build failure by preventing MSVC-specific common compile options (e.g., /wd4996) from propagating into CUDA (.cu) compilation units via extension_cuda’s PUBLIC compile options.

Changes:

• Wrap extension_cuda’s propagated ${_common_compile_options} in a $<$<COMPILE_LANGUAGE:CXX>:...> generator expression so they apply only to C++ compilation.

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[mergennachin]

Land when CI passes