hybrid-cp part 2 for main branch

[xiaoyao0115]

This PR is the second part of hybrid-cp. The first part is: #2282

Compared to part 1, this PR adds the following:​

• Added support for SFT datasets and sequence packing, along with a script. With these additions, hybrid-cp can run end-to-end.​
• Added a mock SFT dataset that lets users control sequence lengths by specifying a sequence-length distribution or by providing a file containing sequence lengths.​
• Migrated the hybrid-cp and sequence packing changes into a dataiterator_wrapper to minimize code changes. Adding a new scheduling algorithm now only requires adding a new scheduler class, which keeps the logic clear and easier to maintain.​
• Added support for FSDP with hybrid-cp; the loss curve is shown below.(model : Qwen3-30B-A3B, hybrid-cp size : 4)
• Added support for PP, but does not support for FSDP+PP.

There's many improvements that we want to make in the future releases.

1. The feature is limited to creating dynamic groups of CP of power 2. We hope to add complete dynamic support using changes in TransformerEngine DPA.
2. The feature does not support CUDA graphs.
3. The feature works best with FlashAttention instead of cuDNN FusedAttention. This is because the changing lengths and CP size make cuDNN recompile the graph and all performance gains are lost. We'll advocate for dynamic support to cuDNN FusedAttention.

Contribution process

flowchart LR
    A[Pre-checks] --> B[PR Tests]
    subgraph Code Review/Approval
        C1[Expert Review] --> C2[Final Review]
    end
    B --> C1
    C2 --> D[Merge]

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[zhujian19891203]

Hi, I am confused, why moe_layer_flops is ignored?

[xiaoyao0115]

Thanks for catching this! I will fix it!