fix(datasets): SUN397 mirror fallbacks and clearer download errors

[Nueramarcos]

Summary

The Princeton SUN397 download URL (vision.princeton.edu/.../SUN397.tar.gz) now 404s after HTTP→HTTPS redirects to cs.princeton.edu. MIT CSAIL mirrors (groups.csail.mit.edu/vision/SUN1old/SUN397.tar) are also down.

This PR:

• Tries HTTPS then HTTP Princeton mirrors (MNIST-style fallback loop)
• Raises an actionable RuntimeError listing failed URLs and pointing users to manual download + Torchvision dataset mirrors #7637 mirroring discussion when all mirrors fail

Fixes #9348

Test plan

• SUN397TestCase (fake data injection) unchanged — no network in unit tests
• Manually verified redirect chain: both mirrors return 404 today; error message now surfaces all attempts

Note

A full fix likely needs an official PyTorch dataset mirror (see #7637). Happy to help host/upload if maintainers want a download.pytorch.org path.

[pytorch-bot]

🔗 Helpful Links

🧪 See artifacts and rendered test results at hud.pytorch.org/pr/pytorch/vision/9528

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