[dev] moe(feat): support chunkwise context parallelism for GDN.

[yuzhongw-nvidia]

What does this PR do ?

PR for main: #6439

FLA gives another CP implementation for GDN fla-org/flash-linear-attention#691, which is claimed to outperform the old A2A-style CP. Integrate the new one into MCore.

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[yuzhongw-nvidia]

/claude review

[claude]

LGTM — the all-gather CP integration and zigzag↔contiguous chunk reshuffling look correct. Left two comments about untested code paths (THD + all-gather CP, and batch > 1 + all-gather CP batch-folding) that may be worth covering or gating.

[Victarry]

LGTM!Thanks for the great work~

[yuzhongw-nvidia]

/ok to test 6aaf4b7

[yuzhongw-nvidia]

/ok to test 0beec47

[xiaoyao0115]

lgtm！

[svcnvidia-nemo-ci]

🔄 Merge queue validation started!

You can track the progress here: https://github.com/NVIDIA/Megatron-LM/actions/runs/28581864199