Add multi-platform torch example

[keith]

This has been a useful starting point for me locally to repro pypi
downloader issues we see in our project.

[keith]

should fail because of #2446

[keith]

ideally after that one is fixed we could make it fail for this issue as well: #2450

currently that takes doing a query though

[aignas]

I am hesitant to add yet another example because we have many examples and not enough CI slots (or at least at some point I've heard that) and most of the cases present in the example already have unit tests, so I am not sure if this is adding anything on top of what is present in bzlmod example.

That said, the pytorch usecase has come up enough times, that I think we should probably update the bzlmod example to use pytorch as part of deps. The only annoying thing being that pytorch wheels are huge and the testing will take longer to just test requirements wiring bits, which is already covered by unit tests as I said previously.

Given that #2488 fixes the original issue, I would be willing to repurpose this work as a section in our docs on using PyPI deps. What do you think?

[keith]

FYI this does not seem to fix #2634, because in that example torch is coming from the same index URL either way