Add Nemotron 3.5 Lightning GB200 nightly test

[Phlip79]

• I, the PR author, have personally reviewed every line of this PR.

What does this PR do?

Adds a 2-node, 8-GPU Nemotron 3.5 Lightning GB200 nightly pretraining test based on Megatron Bridge's nemotron_3_5_lightning_pretrain_8k_config.

Details

• Adds the 20-step, five-repeat TP1/PP1/CP1/EP8 functional test and GB200 golden values.
• Registers the nightly-only dev / dgx_gb200 JET recipe using RP2 and SentencePiece data.
• Sets segment: 2 explicitly so both Slurm nodes remain in the same NVLink domain if the scheduler default changes.
• Extends functional metric extraction and thresholds to validate mtp_2 loss.
• Uses native cross-entropy fusion as the compatibility override for current Megatron argument validation.

Validation

Tested here

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