fix publish to pypi (#439)

Author: LeoXing1996

.github/workflows/publish-to-pypi.yml

@@ -18,6 +18,7 @@ jobs:
           python-version: 3.7
       - name: Build MMGeneration
         run: |
+          pip install torch==1.8.1+cpu torchvision==0.9.1+cpu -f https://download.pytorch.org/whl/torch_stable.html
           pip install wheel
           python setup.py sdist
       - name: Publish distribution to PyPI