CI: only install CPU wheels for torch

.github/workflows/pr.yml

@@ -17,6 +17,7 @@ jobs:
           src: "viscy"
 
   test:
+    name: Test
     needs: [lint]
     runs-on: ubuntu-latest
     strategy:
@@ -30,6 +31,7 @@ jobs:
       - name: Install dependencies
         run: |
           python -m pip install --upgrade pip
-          pip install ".[metrics,dev]"
+          # Install cpu wheels only to speed up the build
+          pip install ".[metrics,dev]" --index-url https://download.pytorch.org/whl/cpu
       - name: Test with pytest
         run: pytest -v