Ensure TRL GRPO imports by installing mergekit

TRL's GRPO trainer path imports mergekit utilities unconditionally in recent releases; without mergekit present the job fails before training starts.\n\nInstall mergekit during bootstrap so GRPOConfig/GRPOTrainer imports succeed in HF Jobs containers.

Constraint: HF Jobs run with ephemeral, system-managed Python (PEP-668)\nRejected: Pin older TRL | risk of drifting from Unsloth-supported TRL range\nConfidence: medium\nScope-risk: narrow\nDirective: If TRL relaxes this dependency again, mergekit can be removed to reduce install time\nTested: python -m py_compile ultimate_sota_training.py\nNot-tested: HF Jobs end-to-end run

ultimate_sota_training.py

@@ -67,6 +67,7 @@ def bootstrap_deps() -> None:
             "httpx>=0.27.0",
             "datasets>=3.4.1,<4.4.0",
             "trl>=0.18.2,<=0.24.0",
+            "mergekit",
             "wandb",
             "matplotlib",
         ]