feat: implement user_agent in layers

[sayakpaul]

Fixes #782

[HuggingFaceDocBuilderDev]

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[github-actions]

Coverage report — kernels/

Measured on: Python 3.10 / Torch 2.13.0.
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: 86.8% — threshold: 80% — ✅

Per-file breakdown

Name	Stmts	Miss	Cover	Missing
src/kernels/__init__.py	14	0	100%
src/kernels/_system.py	6	1	83%	10
src/kernels/_versions.py	78	9	88%	47, 53-54, 57-58, 97, 119, 130, 136
src/kernels/archs.py	56	1	98%	95
src/kernels/backends.py	212	62	71%	40, 44, 48-51, 68, 90, 108, 117, 121, 125-127, 148, 157, 161, 165-167, 188, 199, 201, 208-211, 224, 228, 232-252, 260, 283-303
src/kernels/compat.py	8	1	88%	5
src/kernels/deps.py	81	1	99%	58
src/kernels/hf_hub.py	62	3	95%	18, 20, 114
src/kernels/importer.py	76	6	92%	119, 164, 168, 171, 185-186
src/kernels/install.py	21	7	67%	78-102
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	87	6	93%	91, 122, 194, 314, 340, 368
src/kernels/layer/globals.py	5	0	100%
src/kernels/layer/kernelize.py	74	8	89%	255, 281, 289-290, 296, 300, 316-318
src/kernels/layer/layer.py	217	15	93%	183, 228, 254, 372, 452-453, 465, 474, 482, 493, 522, 526, 539, 592, 622
src/kernels/layer/mode.py	14	0	100%
src/kernels/layer/repos.py	144	42	71%	27, 33, 36-43, 63-64, 70, 73-76, 90, 94, 103-104, 110, 113-116, 123-124, 130, 133-136, 143-144, 150, 153-156, 163-164, 170, 173-176, 257
src/kernels/load.py	69	2	97%	326, 365
src/kernels/locking.py	89	64	28%	35-83, 91-98, 102-125, 137, 152-159, 165-175, 179-186
src/kernels/python_deps.py	58	6	90%	59-60, 64-65, 101, 104
src/kernels/resolver.py	141	2	99%	182, 188
src/kernels/status.py	49	2	96%	23, 81
src/kernels/variants.py	278	19	93%	64, 95, 116, 146, 255-256, 298-301, 303, 387-394, 400-406, 437-443, 455-461
src/kernels/verify.py	88	1	99%	32
TOTAL	2084	276	87%

Updated by the Test kernels workflow on commit e79767526f2edd528194fb43cef67eec7a666fd9.