Looks like this issue is related to virtual environment. Did you try recommended installation line in another/new one virtual environment? If it doesn’t help the possible solution might be installing package using direct link to PyTorch and TorchVision builds for your system:
Windows:
pip install https://download.pytorch.org/whl/cu101/torch-1.4.0-cp38-cp38-win_amd64.whl
pip install https://download.pytorch.org/whl/cu101/torchvision-0.5.0-cp38-cp38-win_amd64.whl
Ubuntu (Linux):
pip install https://download.pytorch.org/whl/cu101/torch-1.4.0-cp38-cp38-linux_x86_64.whl
pip install https://download.pytorch.org/whl/cu101/torchvision-0.5.0-cp38-cp38-linux_x86_64.whl