[Dev] Feat(moe): Gated delta net context parallel (CP)

[yuzhongw-nvidia]

What does this PR do ?

Support context parallel (CP) for Gated delta net (GDN) towards long context training for Qwen3-next series.

MR for main

Methodology

Gated Delta Net (GDN) follows Mamba context parallelization scheme, which adopts all-to-all communication to convert the context parallel for the computation of attention.

Specifically, q, k, v, gate, beta, alpha are initially distributed along the sequence dimension (shape: [s / CP, b, head, ...]). We use all-to-all communication to convert it to head-parallel (shape: [s, b, head / CP, ...]). Then, we can compute the attention parallelly on each CP rank, which has the full sequence but only partial heads. Finally, we use all-to-all communication to convert it back to context-parallel (shape: [s / CP, b, head, ...]).

Usage

Using argument --context-parallel-size.

Convergence

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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Pre-checks

• I want this PR in a versioned release and have added the appropriate Milestone (e.g., Core 0.8)
• I have added relevant unit tests
• I have added relevant functional tests
• I have added proper typing to my code Typing guidelines
• I have added relevant documentation
• I have run the autoformatter.sh on my PR

Code review

The following process is enforced via the CODEOWNERS file for changes into megatron/core. For changes outside of megatron/core, it is up to the PR author whether or not to tag the Final Reviewer team.

For MRs into `main` branch

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2. GitHub auto-assigns expert reviewers based on your changes. They will get notified and pick up your PR soon.

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Final Review might get declined if these requirements are not fulfilled.

(Step 3): Final Review

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(Optional Step 4): Cherry-pick into release branch

If this PR also needs to be merged into core_r* release branches, after this PR has been merged, select Cherry-pick to open a new PR into the release branch.

For MRs into `dev` branch

The proposed review process for `dev` branch is under active discussion.

MRs are mergable after one approval by either eharper@nvidia.com or zijiey@nvidia.com.

Merging your PR

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[yuzhongw-nvidia]

/ok to test 8844935

[yuzhongw-nvidia]

/ok to test 72616c5

[yuzhongw-nvidia]

/ok to test 10f6ab4

[yuzhongw-nvidia]

/ok to test c22d2c3