Avoid Unsloth import in Megatron service

[Kovbo]

Summary

• stop importing ART's Unsloth training stack from art.megatron.service at module import time
• add a local GC/CUDA cleanup helper for MegatronService
• keep create_identity_lora usable in Megatron-only environments without requiring TRL/vLLM side imports

Testing

• uv run --with ruff ruff check src/art/megatron/service.py
• PYTHONPATH=/tmp/art-serverless-fix/src uv run python - <<'PY' from art.megatron.service import create_identity_lora print(create_identity_lora.__name__) PY
• created identity LoRA for Qwen/Qwen3-30B-A3B-Instruct-2507 with rank 1