[algo] feat: Add SVD-LoRA GRPO

[alekseymalakhov11]

What does this PR do?

This PR implements the experimental SVD-LoRA-GRPO method derived by paper ESSA: Evolutionary Strategies for Scalable Alignment

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[gemini-code-assist]

Code Review

This pull request introduces an experimental implementation of SVD-LoRA-GRPO. The core logic is in the new verl/utils/experimental/svd_lora.py file, with integrations in FSDP utilities and worker configurations. My review has identified a few critical issues. There's a TypeError in SVDLinear.create_from_weight that will cause a runtime crash. More importantly, the implementation in apply_svd_lora seems to misinterpret the paper by performing SVD on LoRA's A and B matrices separately, instead of on their product W=BA. I've also pointed out a high-severity issue in fsdp_utils.py where reconstructed weights are not moved to the CPU, and a high-severity maintainability issue regarding the use of a deprecated PyTorch function. Please address these points to ensure the correctness and robustness of this new feature.

[alekseymalakhov11]

We’re implementing SVD-LoRA-GRPO on top of the existing SFT-LoRA adapter. Since #3523 is already in progress and touches the same components, we won’t push our implementation here to avoid conflicts. We’ll wait for #3523 to merge, then rebase and update this PR.

[alekseymalakhov11]

Example training run — Qwen2.5-7B on PRM800K with an SFT-LoRA adapter.
The SFT adapter was trained on a different (non-overlapping) PRM800K training subset.
Hyperparameters:

max_prompt_length = 512
max_response_length = 4096
train_batch_size = 512
lr = 0.01
n = 8
loss_agg_mode = "token-mean"
lora_rank = 16
lora_alpha = 32
target_modules = ["q_proj", "k_proj", "v_proj", "o_proj"]
ppo_mini_batch_size = 64
strategy = "fsdp2"
nnodes = 1
n_gpus_per_node = 8

[alekseymalakhov11]

Hi @eric-haibin-lin @zhaochenyang20 👋
Could you please take a look at my PR when you have a moment?

[alekseymalakhov11]

Hi @eric-haibin-lin @zhaochenyang20 👋
Just a heads-up that the remaining CI failure appears to be unrelated to this PR.
Thanks again for reviewing when you have a moment!