add support for --find-links https://download.pytorch.org/whl/torch_stable.html

[hboutemy]

cited in many places https://stackoverflow.com/questions/60912744/install-pytorch-from-requirements-txt

should be easy to add: no reason to not do it

need to define if it's better to make https://download.pytorch.org/whl/torch_stable.html as as whl/torch_stable.html or whl/simple/torch_stable.html