Feature/gdpo

[kmsgnnew]

Resolves #977

#977

Added implementation of GDPO. Shared below reference link and implementation. This paper improves upon GRPO.

Reference
Reference paper
https://arxiv.org/abs/2601.05242
Reference Implementation
https://github.com/NVlabs/GDPO/blob/b080e63d0126870ad08acc8ebc3f04b728175a9e/trl-GDPO/trl-0.18.0-gdpo/trl/trainer/grpo_trainer.py#L1222

Colab Notebook
NiL

Checklist

• I have added all the necessary unit tests for my change.
• I have verified that my change does not break existing code and all unit tests pass.
• I have added all appropriate doc-strings/documentation.
• My PR is based on the latest changes of the main branch (if unsure, rebase the code).
• I have signed the Contributor License Agreement.
• I have followed Contribution Guidelines.