Refactor async HF writer status files

[caixianzhang]

Summary

• move async HF writer status JSON files into a per-save status directory
• clean up the status directory during successful async HF finalization
• keep the change independent from PR feat: add rank0 async HF save logs #1845

Validation

• ruff check xtuner/v1/model/base.py xtuner/v1/model/compose/base.py
• python3 -m py_compile xtuner/v1/model/base.py xtuner/v1/model/compose/base.py
• git diff --check