ci: use CPU torch on GitHub runners to stop random CUDA-preload segfault

.github/scripts/use_cpu_torch.sh

@@ -0,0 +1,17 @@
+#!/usr/bin/env bash
+# CI runners have no GPU. The CUDA torch build pinned in poetry.lock sometimes
+# segfaults while preloading its CUDA libraries on these runners, so swap in the
+# CPU build of the same torch/torchvision versions after `poetry install`.
+# Usage: use_cpu_torch.sh <python command>, e.g. "python" or "poetry run python".
+set -euo pipefail
+PY=${1:-python}
+
+# Read versions from package metadata; importing torch here could crash.
+pins=$($PY - <<'EOF'
+from importlib.metadata import version
+print(" ".join(f"{p}=={version(p).split('+')[0]}" for p in ("torch", "torchvision")))
+EOF
+)
+$PY -m pip install --no-deps --force-reinstall \
+    --index-url https://download.pytorch.org/whl/cpu $pins
+$PY -c "import torch; assert torch.version.cuda is None, torch.__version__; print('torch', torch.__version__)"

.github/workflows/python-app.yml

@@ -35,6 +35,7 @@ jobs:
         pip install flake8 pytest
         # if [ -f requirements.txt ]; then pip install -r requirements.txt; fi
         poetry install
+        .github/scripts/use_cpu_torch.sh python
     - name: Lint with flake8
       run: |
         # stop the build if there are Python syntax errors or undefined names

.github/workflows/pythonpackage.yml

@@ -28,6 +28,7 @@ jobs:
     - name: Install dependencies
       run: |
         poetry install
+        .github/scripts/use_cpu_torch.sh "poetry run python"
     - name: Format with black
       run: |
         poetry run black .