use cpu-only version torch for workflows

.github/workflows/python-package.yml

@@ -28,7 +28,8 @@ jobs:
       run: |
         python -m pip install --upgrade pip
         python -m pip install flake8 pytest
-        if [ -f requirements.txt ]; then pip install -r requirements.txt; fi
+        pip install numpy numba
+        pip install torch --index-url https://download.pytorch.org/whl/cpu
     - name: Lint with flake8
       run: |
         # stop the build if there are Python syntax errors or undefined names