[training migration] Migrate GPT builder

[maanug-nv]

What does this PR do ?

Follow-up to #4550 for GPT Model.

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[maanug-nv]

TODO:

• remove unnecessary spec function helpers in models/gpt.py
• Ensure no gaps between gpt_builders.gpt_builder() and GPTModelBuilder.build_model()
• implement gpt_config_from_args()
• add unit tests

[maanug-nv]

/claude review

[claude]

Good PR — clean migration of the GPT builder following the established HybridModelBuilder pattern, with solid unit test coverage.

Two inline comments posted:

1. Bug — copy-paste error in GPTModelConfig.__getattr__: Both AttributeError messages say MambaModelConfig instead of GPTModelConfig. This will confuse anyone debugging an attribute lookup failure.

2. Nit — missing trailing comma in __init__.py __all__ list on "GPTModelBuilder".

Everything else (config proxying, vocab padding, layer spec dispatch, MTP handling, pretrain_gpt.py integration, tests) looks correct.

[svcnvidia-nemo-ci]

🔄 Merge queue validation started!

You can track the progress here: https://github.com/NVIDIA/Megatron-LM/actions/runs/26784364022