kernel-builder: reject empty capabilities/archs list

[danieldk]

We compute a kernel component's capabilities by intersecting the capabilities that are specified for the kernel and the capabilities that are supported by CUDA/ROCm.

Before this change, we would silently set an empty list if this intersection was empty. This resulted in CMake falling back to an old capability.

This change fixes that by erroring out when when the capability list is empty.

[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: 83.6% — threshold: 80% — ✅

Per-file breakdown

Name	Stmts	Miss	Cover	Missing
src/kernels/__init__.py	9	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, 79, 95
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	91	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%	156, 199, 205, 214, 306-307, 319, 328, 336, 347, 376, 380, 393, 446, 476
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	69	44	36%	37-98, 102-125
src/kernels/status.py	49	2	96%	23, 81
src/kernels/utils.py	281	54	81%	57, 69-73, 79-80, 205, 209, 212, 274, 299-300, 338, 367, 372, 406, 578, 581, 583, 589, 602-603, 624-631, 635-642, 650, 654-664, 668-675, 713, 717, 736, 738
src/kernels/variants.py	262	19	93%	56, 87, 108, 138, 247-248, 289, 291, 371-378, 384-390, 421-427, 439-445, 533-535
TOTAL	1639	269	84%

Updated by the Test kernels workflow on commit ae44554a3c4fe517e7c7ca92e83fb3706f89dece.