kernel-builder: support SM103 (Blackwell Ultra / B300) capabilities, e.g. 10.3a #720

Context

While porting NATTEN in kernels-community#1031, the NATTEN author suggested adding 10.3a to the Blackwell kernel section's cuda-capabilities to support Blackwell Ultra ([G]B300) — see this review thread. SM103 was added in CTK 12.9 (NATTEN's own arch-by-CTK reference: pkg_helpers.bash).

Problem

kernel-builder currently has no notion of SM103, so declaring 10.3a in a kernel's build.toml is a silent no-op on every build variant:

• cuda_archs_loose_intersection (in src/pyproject/templates/utils.cmake) only keeps an a/f-suffixed arch when its exact base is present in the toolkit's target list, and drops unknown archs without error (only an empty intersection is fatal).
• The target lists in src/pyproject/templates/torch/preamble.cmake contain no 10.3:
  • CTK ≥ 12.8: 7.0;7.2;7.5;8.0;8.6;8.7;8.9;9.0;10.0;10.1;12.0+PTX
  • CTK ≥ 13.0: 7.5;8.0;8.6;8.7;8.9;9.0;10.0;11.0;12.0;12.1+PTX
• src/cuda_supported_archs.json (used by nix-builder/lib/build.nix as the default capability set) also lacks 10.3.

The intersection mechanism itself would otherwise provide exactly the desired gating for free: with 10.3a declared in a section, cu130/cu132 variants would emit sm_103a while cu128 keeps only 10.0a — no per-kernel version conditionals needed.

Proposed change

• Add 10.3 to the CTK ≥ 13.0 branch of CUDA_DEFAULT_KERNEL_ARCHS in preamble.cmake (all current SM103-capable build sets — cu130, cu132 — take this branch; there is no cu129 build set).
• Add 10.3 to src/cuda_supported_archs.json.
• Possibly worth auditing for other missing family members at the same time (e.g. 12.1 is present only in the CTK 13 list; 11.0 likewise).

Kernels can then opt in per section, e.g. NATTEN's Blackwell section:

[kernel.natten_blackwell]
cuda-minver = "12.8"
cuda-capabilities = ["10.0a", "10.3a"]

Impact until fixed

B300 devices still run kernels whose sections emit plain sm_100 SASS (binary-compatible forward within the major), but arch-specific (100a) CUTLASS kernels cannot be used on SM103, so e.g. NATTEN's Blackwell FNA/FMHA paths are unavailable there.

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