Require PyTorch 2.8; build all CUDA 12 wheels against it

[azrael417]

On the v0.9.3rc4 cluster tests, the NCCL of PyTorch 2.6 (2.21.5) and 2.7 (2.26.2) failed the distributed tests:

• H100 (eos), torch 2.6: segfaults in the ring p2p tests on every grid with more than one azimuth rank. On 2×4, almost every test file crashed.
• H100 (eos), torch 2.7: segfaults on 2×4 in test_distributed_primitives and test_distributed_quadrature (collectives only, no p2p), plus a sporadic abort on 4×1.
• B200 (prenyx), torch 2.7: every distributed test process spent about 6 minutes in NCCL.

PyTorch 2.8 (NCCL 2.27.3) and later pass every grid on both clusters, including on eos' driver 535 (CUDA 12.2).

Changes

• Minimum PyTorch is 2.8 (pyproject.toml: torch>=2.8.0).
• cu126 and cu128 are now built against PyTorch 2.8.0 instead of 2.6/2.7, alongside cu129. Every CUDA 12 variant runs on the NCCL that passed, and each matches the CUDA build of torch 2.8 its users have; plain pip install torch==2.8.0 is the cu128 build. Still 7 variants, 24 build jobs.
• README / docs/install.md: users of PyTorch before 2.8 or CUDA before 12.6 are pointed to the Docker container, or to a source build with --no-deps. Without --no-deps, the torch>=2.8 requirement makes pip replace their PyTorch. Those combinations are untested.
• Changelog: breaking entry, and the section is relabelled ### v0.9.3rc5 for the next tag.

🤖 Generated with Claude Code

[greptile-apps]

Confidence Score: 5/5

[High risk] Bumps minimum PyTorch version and rebuilds all CUDA 12 wheels.

The PR appears safe to merge; no blocking issue remains from this review.

Summary

Raises the minimum PyTorch version to 2.8 and builds all three CUDA 12 wheel variants against PyTorch 2.8.0.

• Keeps the CUDA 12.6 and 12.8 packages, addressing the earlier installation-guidance concern.
• Documents Docker and untested source-build options for older installations.
• Adds explicit targets for the source-build and Docker links in docs/install.md.
• No new actionable issues were found in the changes since the previous review.

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