fix: benchmark_model CPU timing units

[AdudodlaVarish]

Fix benchmark_model CPU timing units

benchmark_model() returns milliseconds on CUDA, MPS, and XPU, but returns seconds on CPU. This makes CPU results numerically wrong by a factor of 1,000.

Convert the CPU elapsed time to milliseconds and document the return unit. Add a deterministic test using a real CPU Linear module and a controlled clock: two runs over a 0.5-second interval must return 250 milliseconds per run, rather than 0.25.

Validation on Windows, Python 3.12.14, PyTorch 2.14.1+cpu:

python -m pytest test/test_utils.py -q
Before: 1 failed, 9 passed, 3 skipped
After: 10 passed, 3 skipped

Targeted Ruff lint (--select F,I) and format checks pass (Ruff 0.16.10), as does git diff --check. The full test suite, full pre-commit, and accelerator execution were not run.

[pytorch-bot]

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