Derive the autocast test backend and scope the bf16 gate to NCCL

[delock]

Description

bf16_required_version_check() (tests/unit/util.py) requires torch >= 1.10, CUDA >= 11.0 and NCCL >= 2.10.3. On the cpu accelerator, bf16 collectives run over gloo/ccl and none of those transport dependencies exist, so the check always returns False and every bf16 test is skipped — about 40 call sites across 15 files. test_zero_autocast.py is worse off: it raises instead of skipping, so each of its cases counts as a failure (24 on the multi-rank CPU run in #8381).

This PR scopes the version floors inside the check itself:

if (cpu_accelerator and accelerator_pass) or (torch_version_available and cuda_version_available
                                              and nccl_version_available and accelerator_pass):
    return True

• cpu: only the accelerator's own bf16 support (is_bf16_supported()) is required — the torch/CUDA/NCCL floors are transport dependencies that gloo/ccl does not have
• every other accelerator (cuda, npu, hpu, xpu, mlu, ...): evaluates the exact original floors; cpu_accelerator is False so the expression is bit-identical to before, and the npu/hpu/xpu exemption branches are untouched
• test_zero_autocast.py: the bf16 gate goes back to the bare call every other caller uses, and skips instead of raising

The hardcoded init_distributed(dist_backend='nccl') in the same test is deliberately left alone: it is a no-op (the harness already initialized the process group, comm.py:838-839), and deriving the backend per accelerator would change behavior on non-cuda accelerators (npu/hpu resolve to hccl etc.). The baseline DDP(device_ids=[i]) pinning is left for a follow-up (#8399 fixed the same pattern elsewhere).

Validation (executed on real hardware)

• 20-core x86_64 CPU, torch 2.13.0+cpu, gloo backend
• Direct call: bf16_required_version_check() on cpu returns False before, True after. Note: CPU_Accelerator.is_bf16_supported() is currently a stub that always returns True, so the cpu path does not gate on the hardware's bf16 instructions — giving it a real capability probe is left as a follow-up
• Non-cpu equivalence: with cpu_accelerator == False the new expression reduces exactly to the original A and C and N and P
• pre-commit (yapf / flake8 / check-torchdist / codespell) passes on both changed files

Sibling PRs from the same series: #8397, #8398, #8399, #8407, #8409. Exposed by the LOCAL_SIZE=4 multi-rank CPU run in #8381.