download cpu-only pytorch during docs build for speedup

Author: rfeinman

.github/workflows/documentation.yml

@@ -27,7 +27,7 @@ jobs:
       - name: Install dependencies
         run: |
           python -m pip install --upgrade pip
-          python -m pip install -r requirements.txt
+          python -m pip install -r requirements.txt --extra-index-url https://download.pytorch.org/whl/cpu
           python -m pip install -r docs/requirements.txt
 
       - name: Sphinx build