Key the doc build concurrency group on the pull request number

[albertvillanova]

What does this PR do?

build_pr_documentation.yaml keys its concurrency group on github.head_ref, which is the head branch name without the fork owner. Two open PRs from different forks whose branches share a name therefore land in the same group, and a push on one cancels the in-flight doc build on the other.

Shared names are common, because many contributors push to their fork's default branch instead of creating one. No two of the 12 currently open PRs here share a head branch name, but 4 of them come from forks, so the collision is reachable, and a doc build takes around a minute, which is the window in which it would cancel a build.

github.event.pull_request.number is unique per pull request. This workflow only triggers on pull_request, so the github.run_id fallback is never reached, but keeping it means that a later trigger would fall back to never cancelling rather than to grouping by branch name.

The template this workflow was copied from carries the same expression, and the same one-liner is proposed there in huggingface/doc-builder#829.

[github-actions]

Hi @albertvillanova, thanks for your interest in contributing!

This project requires that pull request authors are vouched, and you are not in the list of vouched users.

This PR will be closed automatically. See https://github.com/huggingface/kernels/blob/main/CONTRIBUTING.md for more details.

[HuggingFaceDocBuilderDev]

The docs for this PR live here. All of your documentation changes will be reflected on that endpoint. The docs are available until 30 days after the last update.

[albertvillanova]

CC: @danieldk @sayakpaul

[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: 87.1% — 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%	48, 54-55, 58-59, 98, 120, 131, 137
src/kernels/archs.py	56	1	98%	94
src/kernels/backends.py	213	62	71%	42, 46, 50-53, 70, 92, 110, 119, 123, 127-129, 150, 159, 163, 167-169, 190, 201, 203, 210-213, 226, 230, 234-254, 262, 285-305
src/kernels/compat.py	8	1	88%	5
src/kernels/deps.py	65	1	98%	58
src/kernels/hf_hub.py	63	2	97%	20, 22
src/kernels/importer.py	44	5	89%	82, 86, 89, 103-104
src/kernels/install.py	21	7	67%	76-100
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	88	6	93%	92, 125, 198, 318, 344, 374
src/kernels/layer/globals.py	5	0	100%
src/kernels/layer/kernelize.py	80	8	90%	258, 293, 301-302, 308, 312, 328-330
src/kernels/layer/layer.py	218	14	94%	189, 236, 262, 396, 476-477, 498, 506, 517, 546, 550, 563, 616, 646
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	70	2	97%	329, 368
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	156	2	99%	220, 226
src/kernels/status.py	50	2	96%	25, 79
src/kernels/validate.py	54	1	98%	85
src/kernels/variants.py	278	19	93%	65, 96, 117, 147, 256-257, 299-302, 304, 388-394, 400-406, 437-443, 455-461
src/kernels/verify.py	88	1	99%	33
TOTAL	2117	274	87%

Updated by the Test kernels workflow on commit a877ba1f2b318811b36c702c02a03dea562176f5.