Document the "scipy" extra

[IRDonch]

Torchvision's setup.py defines a scipy extra that pulls in the scipy dependency that is needed by some of Torchvision's modules. This lets you use torchvision[scipy] as an argument to pip install, which is good, since it's a way to explicitly state that you're installing SciPy as a dependency of Torchvision.

However, this extra doesn't seem to be documented anywhere, so it's unclear whether doing pip install torchvision[scipy] is officially supported. It would be nice if this was documented in the README or elsewhere.

[fmassa]

I agree. We haven't documented scipy (or pycocotools) because they are not hard-requirements, and only a few functions of torchvision depends on them.

But it might be good to document that somewhere, maybe in the functions that use them?

Can you send a PR improving the documentation?

[nvs-abhilash]

@fmassa I have created a PR #1966 for this issue. Please let me know if anything else is required.