[Dev] Optimizer State and Master Weight Offloading

[hxbai]

What does this PR do ?

main branch: #2811

PR Description: Optimizer State Offloading for DistributedOptimizer

Summary

This PR introduces optimizer state offloading to CPU for the DistributedOptimizer, enabling significant GPU memory savings during training by temporarily moving optimizer states (exp_avg, exp_avg_sq) and master weights to CPU memory when not in use.

Motivation

During the forward and backward passes, optimizer states occupy GPU memory but are not actively used. For large models, these states can consume a substantial portion of GPU memory. By offloading optimizer states to CPU after optimizer.step() and reloading them before the next step, we can reclaim this GPU memory for other operations like activation checkpointing or larger batch sizes.

Performance & Memory Savings

Memory Savings:

• On DeepSeek-V3, this feature saves 15-20GB of GPU memory (with 0.1-0.2s/iter overhead on GB200)

Comparison with Optimizer CPU Offloading:

Aspect	CPU Offloading (ZeRO-Offload style)	State Offloading (This PR)
Where optimizer runs	CPU	GPU
D2H/H2D frequency	Every step (gradients + params)	Every step (states only)
Compute location	Adam step on CPU	Adam step on GPU
Best for	Memory-constrained, bandwidth-limited	High-bandwidth interconnects (NVLink, GB200)

More details:

• With higher H2D/D2H bandwidth such as GB200, the state offloading has significantly less overhead
• Async transfer overlapping
• Optimizer step still runs on GPU, avoiding CPU compute bottleneck
• Pinned memory enables maximum PCIe/NVLink bandwidth utilization
• Currently requires TE FusedAdam optimizer

⚠️ For major changes (either in lines of code or in its impact), please make sure to first share discuss a design-doc with the team.

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

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

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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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[hxbai]

/ok to test b3f0ab3

[hxbai]

/ok to test a27aa49

[hxbai]

/ok to test 3ec46ba

[hxbai]

/ok to test d2e4773

[hxbai]

/ok to test 39d5ba5

[hxbai]

/ok to test cea4340

[hxbai]

/ok to test cecd4fc