Fix Windows unittest CI: force CPU-only build to avoid CUDA DLL load failure

The Windows CI image ships CUDA toolkits on PATH. After adding (13, 2) to
SUPPORTED_CUDA_VERSIONS (#20440), install_executorch's auto-detection
(setup.py: is_cuda_available() via nvcc) started returning True on the Windows
runner (which has the CUDA 13.2 toolkit), so it flipped EXECUTORCH_BUILD_CUDA=ON.
But the unittest jobs install CPU torch, so the resulting CUDA build of
_portable_lib fails to load its CUDA DLLs at import time:

  ImportError: DLL load failed while importing _portable_lib

causing all pytest collection to error out (unittest / unittest-editable /
unittest-release on windows).

Add a -cpuOnly switch to setup-windows.ps1 that forces
-DEXECUTORCH_BUILD_CUDA=OFF via CMAKE_ARGS, and pass it from the CPU unittest
workflow. The CUDA Windows jobs (cuda-windows.yml) keep the default and are
unaffected.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

Author: gasoonjia

.ci/scripts/setup-windows.ps1

@@ -1,5 +1,6 @@
 param (
-    [string]$editable = "false"
+    [string]$editable = "false",
+    [string]$cpuOnly = "false"
 )
 
 conda create --yes --quiet -n et python=3.12
@@ -13,6 +14,16 @@ conda activate et
 # Install test dependencies
 pip install -r .ci/docker/requirements-ci.txt
 
+# The Windows CI image ships CUDA toolkits on PATH, so install_executorch
+# (setup.py) auto-enables EXECUTORCH_BUILD_CUDA whenever the detected nvcc
+# version is in SUPPORTED_CUDA_VERSIONS. CPU-only jobs install CPU torch, so a
+# CUDA build of _portable_lib then fails to load its CUDA DLLs at import time
+# ("DLL load failed while importing _portable_lib"). Force a CPU-only build
+# when the caller asks for it.
+if ($cpuOnly -eq 'true') {
+    $env:CMAKE_ARGS = "$env:CMAKE_ARGS -DEXECUTORCH_BUILD_CUDA=OFF"
+}
+
 if ($editable -eq 'true') {
     install_executorch.bat --editable
 } else {

.github/workflows/_unittest.yml

@@ -72,7 +72,7 @@ jobs:
           \$ErrorActionPreference = 'Stop'
           \$PSNativeCommandUseErrorActionPreference = \$true
 
-          .ci/scripts/setup-windows.ps1 -editable "${{ inputs.editable }}"
+          .ci/scripts/setup-windows.ps1 -editable "${{ inputs.editable }}" -cpuOnly true
           if (\$LASTEXITCODE -ne 0) {
               Write-Host "Setup failed. Exit code: \$LASTEXITCODE."
               exit \$LASTEXITCODE