Implement Speculative transform script for GPT models

[AAnoosheh]

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What does this PR do ?

Adds a conversion script to prepare a model for Speculative Decoding via Nvidia Model Optimizer.

Collection: LLM

Changelog

• Add a model-transform callable specifically for converting a given NeMo 2 model to a Model Optimizer Speculative Decoding model.
• Add a script to conveniently load an existing checkpoint, call the transform, and save the model.

Usage

python scripts/llm/gpt_speculate.py \
    --model_path $CKPT_PATH \
    --num_eagle_layers 1 \
    --export_dir $SAVE_PATH \

    llm.train(
        model=model,
        data=data,
        trainer=trainer,
        ...,
        model_transform=SpeculativeTransform(
            num_eagle_layers=args.num_eagle_layers,
            num_medusa_heads=args.num_medusa_heads,
        ),
    )

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• [ ] Bugfix
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Additional Information

[AAnoosheh]

NOTE: Depends on https://github.com/NVIDIA/NeMo/pull/13599

[github-actions[bot]]

[🤖]: Hi @AAnoosheh 👋,

We wanted to let you know that a CICD pipeline for this PR just finished successfully.

So it might be time to merge this PR or get some approvals.

//cc @chtruong814 @ko3n1g @pablo-garay @thomasdhc