As/torch

[AmitSolomonPrinceton]

Improvements to torch.py, including parallelization, refactoring, update existing solvers instead of recreating them. Also added devcontainer, set the version of scipy to <1.12, and update build files.

[vineetbansal]

Great to see that the CI is working on this PR!

I have a couple of suggestions, mostly having to do with my taking a shortcut on this before, something that might be handled better with this PR (and going forward):

• My reason for not introducing torch as a package dependency was that its a heavy and complex dependency, probably applicable to a small majority of osqp users. Users of the nn submodule may as well see an ImportError on torch and know what they're supposed to do. Now that we have joblib joining that list as well. it may be better something like a [nn] extra so that pip install osqp[nn] installs the dependencies required for the nn functionality.
• Alternatively, since joblib is a small and generally useful library, it can be introduced as a direct dependency, and torch left out as before.
• In either case, I think moving away from installing stuff in CIBW_TEST_REQUIRES will keep things manageable going forward. (and the CI can simply do a install osqp[dev,nn,..] etc.

I also don't recommend including .devcontainer in this PR, since that might trigger an IDE-specific proliferation of code, but that's just my personal opinion.