kernel-builder: support specifying backend archs for noarch kernels

[danieldk]

So far, we have assumed that noarch kernels work on all GPU architectures for a supported backend. However, some noarch kernels work on a more limited set of hardware. This change adds options to the torch-noarch section to indicate which archs are supported, which are then written to the kernel metadata.

Ideally we'd keep this path the same as arch kernels, where the capabilities are computed and written to metadata at build-time. However, this turns out to be quite tedious.

This PR lets kernel-builder write it to metadata, but keeps noarch vs arch handling out of the common paths, keeping the functions in common as generic is possible.

Instead, we move some of the code to the kernels-data crate. The Metadata struct gets a new for_backend constructor that creates the metadata for a given build configuration and backend.

[HuggingFaceDocBuilderDev]

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[github-actions]

Coverage report — kernels/

Measured on: Python 3.10 / Torch 2.12.0.
Other CI configurations are not included in this number.
Hardware-gated code paths (ROCm/XPU/NPU/Darwin/Windows) are excluded or unreachable on the Linux+CUDA runner.

Total coverage: 84.4% — threshold: 80% — ✅

Per-file breakdown

Name	Stmts	Miss	Cover	Missing
src/kernels/__init__.py	10	0	100%
src/kernels/_system.py	6	1	83%	10
src/kernels/_versions.py	63	7	89%	46, 49, 52-53, 56-57, 100
src/kernels/backends.py	194	55	72%	40, 44, 48-51, 68, 90, 108, 117, 121, 125-127, 148, 170, 181, 188-191, 201, 205-225, 233, 256-276
src/kernels/compat.py	8	1	88%	5
src/kernels/deps.py	54	4	93%	58-59, 95, 98
src/kernels/layer/__init__.py	6	0	100%
src/kernels/layer/_interval_tree.py	103	4	96%	23, 52, 147, 150
src/kernels/layer/device.py	48	14	71%	42, 47-49, 91, 96-98, 101, 149, 152, 155-157
src/kernels/layer/func.py	93	7	92%	72, 100, 154, 257, 263, 272, 290
src/kernels/layer/globals.py	5	0	100%
src/kernels/layer/kernelize.py	73	8	89%	255, 273, 281-282, 288, 292, 308-310
src/kernels/layer/layer.py	174	15	91%	166, 209, 215, 228, 320-321, 333, 342, 350, 361, 390, 394, 407, 460, 490
src/kernels/layer/mode.py	14	0	100%
src/kernels/layer/repos.py	130	34	74%	27, 33, 36-41, 61-62, 68, 71-74, 88, 92, 101-102, 108, 111-114, 121-122, 128, 131-134, 141-142, 148, 151-154, 235
src/kernels/lockfile.py	71	46	35%	37-104, 108-131
src/kernels/status.py	49	2	96%	23, 81
src/kernels/utils.py	301	55	82%	65, 77-81, 87-88, 218, 222, 225, 287, 295, 334-335, 373, 404, 409, 444, 673, 676, 678, 684, 697-698, 719-731, 735-742, 750, 754-764, 768-775, 813, 817, 836, 838
src/kernels/variants.py	262	19	93%	56, 87, 108, 138, 247-248, 289, 291, 371-378, 384-390, 421-427, 439-445, 534-536
src/kernels/verify.py	88	1	99%	32
TOTAL	1752	273	84%

Updated by the Test kernels workflow on commit e0ab830ec91514985da74018fc8190ae1476720a.