refactor(fsdp): rename sleep_wake.py to device_move.py

[Rockdu]

Stack 8/9 of follow-ups to #263: main ← #265 ← #266 ← #267 ← #268 ← #269 ← #271 ← #272 ← this ← #274.

What

• miles/backends/fsdp_utils/sleep_wake.py becomes device_move.py, named after what it holds (move_model, move_optimizer, and the frozen-model sleep/wake built on them). Its GPU test and worker follow: test_device_move.py, _device_move_worker.py.

Why

Other RL trainers name these helpers after the move (offload_fsdp_model_to_cpu in verl, move_to_device in NeMo-RL, move_torch_optimizer in miles); sleep / wake_up stay the actor's phase names, as in miles and slime. Renaming here instead of in #267 keeps the CI results of the PRs below.

Checklist

• pre-commit run passes on the changed files
• Added/updated tests for new behaviour — rename only
• pytest -x is green — CPU fsdp tests pass locally; the renamed GPU test is still registered on stage-b-5-gpu-h200
• If launch flags changed, python3 train.py --help still parses — n/a
• If a public flag was added, it appears in the CLI reference docs — n/a
• If an example was added, it has a real walkthrough — n/a