Megatron-FSDP: Fix insufficient double buffers during gradient reduce

[shjwudp]

What does this PR do ?

Fix the issue where the gradient double buffer runs out.

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[cspades]

Can you test it with wgrad accum fusion as well? The error should happen on Llama 8B MBS4 on 1 GB300 node using pure FSDP. Thanks!

[shjwudp]

Can you test it with wgrad accum fusion as well? The error should happen on Llama 8B MBS4 on 1 GB300 node using pure FSDP. Thanks!

@cspades I’ve tested this on Qwen-30B MoE, and it works fine (it can be enabled together with gradient accumulate fusion). 99% sure this patch will fix your issue.

[cspades]

/ok to test 2888056

[cspades]

/ok to test 74edeeb

[svcnvidia-nemo-ci]

🔄 Merge queue validation started!

You can track the progress here: https://github.com/NVIDIA/Megatron-LM/actions/runs/23876361801

[svcnvidia-nemo-ci]

🔄 Merge queue validation started!

You can track the progress here: https://github.com/NVIDIA/Megatron-LM/actions/runs/23877163131