Context

Part of #4223 and #4374 - Moving trainers to experimental submodule for V1.

Task

Move GKDTrainer from main trl module to trl.experimental:

• Move trainer file to trl/experimental/
• Update imports in init.py files
• Update documentation
• Add deprecation warning in old location
• Update tests
• Verify examples still work

Post-V1 Plan

May be removed based on usage and maintenance requirements.

Related

• Parent tracking issue: Road to v1 #4374
• RFC: [RFC] Moving Most TRL Trainers to the experimental Submodule to Streamline the Core #4223
• BCO migration (completed): 🚚 Move BCO to trl.experimental #4312