feat: Update mbridge with cache support

[ZhiyuLi-Nvidia]

What does this PR do ?

This is to add cache support in mbridge submodule, which mitigates the dsv3 refitting regression:

• dsv3: from ~70s to ~55s here
  • with another memory fix we should expect to get 43s best here
• moonshort 16B: 2x speedup
  No need to change the .gitmodules since we updated to the same branch yifu/nemo-rl-use-chunkpatch-ds
  Add cache to optimize refit performance Megatron-Bridge#621

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Additional Information

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Summary by CodeRabbit

• Chores
  • Updated a third-party submodule to the latest revision to keep dependencies current.
  • No changes to app functionality, UI, or public APIs.
  • No configuration or migration required for end-users.
  • This update is intended to maintain alignment with upstream without altering behavior.
  • Expect no impact on performance, features, or compatibility.

[github-actions]

✅ Submodule Fast-Forward Check Results

Check based on commit: 0f35de4 (PR #1187 from zhiyul/cache_in_mbridge)

✅ Submodules that are properly updated:

Megatron-Bridge: ✅ PR branch is ahead of main branch (fast-forward)

All submodule changes look good! ✨

[ZhiyuLi-Nvidia]

@terrykong @guyueh1 could you take a review?

[coderabbitai]

📝 Walkthrough

Walkthrough

Updated the Git submodule reference for 3rdparty/Megatron-Bridge-workspace/Megatron-Bridge to a new commit SHA. No code or API changes in this repository.

Changes

Cohort / File(s)	Summary
Submodule pointer update 3rdparty/Megatron-Bridge-workspace/Megatron-Bridge	Advance submodule from abd52c8 to 85a37ff; no interface or public API changes noted.

Estimated code review effort

🎯 1 (Trivial) | ⏱️ ~2 minutes

Pre-merge checks and finishing touches

❌ Failed checks (1 inconclusive)

Check name	Status	Explanation	Resolution
Test Results For Major Changes	❓ Inconclusive	Based on the PR summary, this is a submodule bump that introduces cache support in Megatron-Bridge with claimed performance improvements and links to Weights & Biases runs; however, there are no explicit test results, numerics/convergence checks, or detailed before/after metrics with configuration captured in the provided description beyond high-level timing claims. Since this change likely affects performance and possibly numerics via caching behavior, the PR should document concrete benchmarks (setup, model/config, hardware, dataset, and exact metrics) and any convergence validation. Given the absence of such testing details in the provided context, the requirement to include test results for major/performance-impacting changes is not met.	Please update the PR description to include: (1) links or inline summaries of benchmark runs with exact configs (model, batch size, sequence length, hardware, software versions), (2) before/after timing or throughput tables for the affected paths (e.g., dsv3 refit and moonshort 16B) and (3) a brief numerics/convergence check showing no regression. If such details already exist in the PR body or linked Megatron-Bridge PR, surface them explicitly here to satisfy the check.

✅ Passed checks (3 passed)

Check name	Status	Explanation
Title Check	✅ Passed	The title "feat: Update mbridge with cache support" is concise, follows commit-style conventions, and accurately summarizes the primary change (updating the mbridge/Megatron-Bridge submodule to add cache support) described in the PR summary and raw changes. It is specific and clear enough for a teammate scanning history and does not include noise or ambiguous wording.
Docstring Coverage	✅ Passed	No functions found in the changes. Docstring coverage check skipped.
Description Check	✅ Passed	Check skipped - CodeRabbit’s high-level summary is enabled.

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Actionable comments posted: 0

🧹 Nitpick comments (1)

3rdparty/Megatron-Bridge-workspace/Megatron-Bridge (1)

1-1: Document/cache toggle and rollback plan.

Cache support in the submodule can change behavior/perf. Please add a brief CHANGELOG/release note and mention any flags/env vars to disable it for rollback if needed; link the W&B runs for reproducibility.

I can draft the release-note snippet if helpful.

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📥 Commits

Reviewing files that changed from the base of the PR and between 42aa41b and 0f35de4.

📒 Files selected for processing (1)

• 3rdparty/Megatron-Bridge-workspace/Megatron-Bridge (1 hunks)

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🔇 Additional comments (2)

3rdparty/Megatron-Bridge-workspace/Megatron-Bridge (2)

1-1: Submodule pin verified — no action required. 3rdparty/Megatron-Bridge-workspace/Megatron-Bridge is pinned to 85a37ffdf02edc07c0a7ac97cb9abcafcd0ac0ed (merge commit of PR #621) and that commit is the head of branch yifu/nemo-rl-use-chunkpatch-ds; .gitmodules has no branch configured, so the SHA pin is explicit and reproducible.

---

1-1: Ensure CI fetches submodules recursively (actions/checkout: submodules: true/recursive, fetch-depth: 0)
No .github/workflows/* files were found during verification — confirm your CI (GitHub Actions or other) is configured to fetch submodules (actions/checkout with submodules: true/recursive and fetch-depth: 0) or update CI accordingly.

[github-actions]

✅ Submodule Fast-Forward Check Results

Check based on commit: 583a283 (PR #1187 from zhiyul/cache_in_mbridge)

✅ Submodules that are properly updated:

Megatron-Bridge: ✅ PR branch is ahead of main branch (fast-forward)

All submodule changes look good! ✨

[ZhiyuLi-Nvidia]

@yaoyu-33 could you take a review as well?

[github-actions]

✅ Submodule Fast-Forward Check Results

Check based on commit: a822dcd (PR #1187 from zhiyul/cache_in_mbridge)

✅ Submodules that are properly updated:

Megatron-Bridge: ✅ PR branch is ahead of main branch (fast-forward)

All submodule changes look good! ✨