Move GKDTrainer to experimental module

[behroozazarkhalili]

Summary

This PR migrates GKDTrainer and GKDConfig from trl.trainer to trl.experimental.gkd as part of the TRL V1 refactoring effort.

Resolves #4462
Related to #4374 (Road to v1)
Related to #4223 (Experimental trainers RFC)

Changes Made

Module Structure

• ✅ Created trl/experimental/gkd/ module with __init__.py, gkd_config.py, and gkd_trainer.py
• ✅ Moved GKDTrainer (450 lines) and GKDConfig (113 lines) to experimental location
• ✅ Updated imports in experimental version (relative path adjustments from .. to ...)
• ✅ Removed TRL_EXPERIMENTAL_SILENCE warning (no longer needed in experimental module)

Backward Compatibility

• ✅ Replaced original files with deprecation stubs that inherit from experimental versions
• ✅ Added FutureWarning in both __init__ and __post_init__ methods
• ✅ Deprecation notice references TRL 0.29 removal and issue [RFC] Moving Most TRL Trainers to the experimental Submodule to Streamline the Core #4223
• ✅ Maintains full backward compatibility until TRL 0.29

Tests & Examples

• ✅ Updated tests/test_gkd_trainer.py to import from trl.experimental.gkd
• ✅ Updated examples/scripts/gkd.py to import from experimental location

Documentation

• ✅ Updated docs/source/gkd_trainer.md with new import examples
• ✅ Moved GKD from Trainers to Experimental section in docs/source/_toctree.yml
• ✅ Updated docs/source/reducing_memory_usage.md import example
• ✅ Updated docs/source/liger_kernel_integration.md import example

Migration Path

Before (deprecated, will be removed in TRL 0.29):

from trl import GKDConfig, GKDTrainer

After (recommended):

from trl.experimental.gkd import GKDConfig, GKDTrainer

Testing

• ✅ All existing tests updated to use new import path
• ✅ Backward compatibility maintained through deprecation stubs
• ✅ Example scripts updated and verified

Checklist

• Module structure created in trl/experimental/gkd/
• Deprecation stubs added in original locations
• Tests updated
• Examples updated
• Documentation updated
• Backward compatibility maintained
• Follows established migration pattern (BCO, CPO, OnlineDPO)

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[qgallouedec]

lgtm, thanks!