[Dev] fix(megatron-fsdp): Resolve hang caused by non-deterministic reduce-scatter

[xuwchen]

What does this PR do ?

main PR: #2218

Fix hang caused by non-deterministic reduce-scatter operations.

Megatron-FSDP uses a set container to manage gradients, resulting in inconsistent gradient reduce ordering across DP ranks. This leads to different bucket execution orders during reduce-scatter, and when buckets have varying sizes, distributed operations will hang.

This issue has a low probability of occurring and is typically triggered only in certain model configurations. Setting TORCH_DISTRIBUTED_DEBUG=DETAIL can help identify instances of this problem.

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.

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

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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 [email protected] or [email protected].

Merging your PR

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[xuwchen]

/ok to test 54b10ff

[ko3n1g]

This caused a performance regression on 8x7b. Please check the internal messages for a follow-up