[Bugfix][CPU] Make the Apple Silicon BF16 probe fall back instead of raising

[UgaTheDev]

Problem

In CpuPlatform.supported_dtypes, the macOS arm64 branch probes for BF16 with:

if (
    subprocess.check_output(
        ["sysctl -n hw.optional.arm.FEAT_BF16"], shell=True
    ).strip()
    == b"1"
):
    return [torch.bfloat16, torch.float16, torch.float32]
return [torch.float16, torch.float32]

macOS does not publish the hw.optional.arm.FEAT_BF16 OID when the CPU does not
have the feature. sysctl then prints unknown oid to stderr and exits 1.
check_output raises CalledProcessError on a nonzero exit, so it never returns
a value other than b"1", and the return [torch.float16, torch.float32] line
below it cannot be reached whenever the OID is absent. That fallback is the
fp16/fp32 baseline the Apple docs describe: "Currently the CPU implementation for
macOS supports FP32 and FP16 datatypes"
(docs/getting_started/installation/cpu.apple.inc.md).

supported_dtypes is read during startup from vllm/config/model.py, so the
exception would propagate out of config resolution rather than selecting fp16.

I do not have hardware without FEAT_BF16 to confirm which specific machines and
macOS versions omit the OID, so this is reported as an unreachable branch rather
than as a reproduced crash on any particular model.

Fix

Use subprocess.run without check=True, so a nonzero exit returns normally and
falls through to fp16/fp32:

bf16 = (
    subprocess.run(
        ["sysctl", "-n", "hw.optional.arm.FEAT_BF16"], capture_output=True
    ).stdout.strip()
    == b"1"
)

This also drops shell=True on a single-element list, which was passing the whole
command string as one argv element and relying on the shell to split it.

Testing

Checked the two subprocess calls directly on macOS 15 arm64:

• absent OID (hw.optional.arm.FEAT_NOPE): check_output raises
  CalledProcessError; run returns returncode=1 with empty stdout, so the
  comparison is False and the fp16/fp32 branch is taken.
• present OID (hw.optional.arm.FEAT_BF16): returncode=0, stdout b"1", so the
  bf16 branch is taken.

CpuPlatform().supported_dtypes on this machine returns
[torch.bfloat16, torch.float16, torch.float32], unchanged from before.

[claude]

Claude Code Review

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[bigPYJ1151]

/ci run

[github-actions]

✅ Triggered Buildkite CI #83269 for commit cb3ba157cbed.