What changed

• ♻️ Fix caching in SFT by @qgallouedec in #2945
• 🐯 Fix LigerKernel for SFTTrainer by @lewtun in #2940
• 📌 Pin liger-kernel and vLLM by @qgallouedec in #2952

Full Changelog: https://github.com/huggingface/trl/compare/v0.15.1...v0.15.2