Make GPU tests safe to collect on CPU-only hosts

[ZhiyaoWen999]

Summary

• Defer accelerator discovery for GPU-marked tests to a pytest fixture.
• Build device-dependent rank cases at test runtime instead of collection time.
• Initialize device managers inside worker processes.
• Skip GPU/NPU tests cleanly when the required accelerator is unavailable.

Root cause

GPU/NPU test modules initialized DeviceManager and evaluated device counts during module import and parameter collection. On CPU-only hosts, this could fail before pytest applied the -m "not gpu" filter.

Validation

• pytest -v -m "not gpu" tests — 55 passed, 1 skipped, 6 deselected
• pytest -q tests — 55 passed, 7 skipped
• ruff check .
• ruff format --check .

Closes #95