[trainer] feat: Enabled fused adamw

[puneeshkhanna]

What does this PR do?

Enable fused adamw which should be generally faster and more memory efficient.
Also provide the config parameter to set eps.

Checklist Before Starting

• Search for similar PRs. Paste at least one query link here: ...
• Format the PR title as [{modules}] {type}: {description} (This will be checked by the CI)
  • {modules} include fsdp, megatron, sglang, vllm, rollout, trainer, ci, training_utils, recipe, hardware, deployment, ray, worker, single_controller, misc, perf, model, algo, env, tool, ckpt, doc, data
  • If this PR involves multiple modules, separate them with , like [megatron, fsdp, doc]
  • {type} is in feat, fix, refactor, chore, test
  • If this PR breaks any API (CLI arguments, config, function signature, etc.), add [BREAKING] to the beginning of the title.
  • Example: [BREAKING][fsdp, megatron] feat: dynamic batching

Test

In our internal code base, we always set fused adamw to True and works fine. However right now, I don't have the step time comparison with and without it.
At the same time, I can push same change in RL code too and setting to True should be generally beneficial.

API and Usage Example

Demonstrate how the API changes if any, and provide usage example(s) if possible.

# Add code snippet or script demonstrating how to use this

Design & Code Changes

Demonstrate the high-level design if this PR is complex, and list the specific changes.

Checklist Before Submitting

Important

Please check all the following items before requesting a review, otherwise the reviewer might deprioritize this PR for review.

• Read the Contribute Guide.
• Apply pre-commit checks: pre-commit install && pre-commit run --all-files --show-diff-on-failure --color=always
• Add / Update the documentation.
• Add unit or end-to-end test(s) to the CI workflow to cover all the code. If not feasible, explain why: ...
• Once your PR is ready for CI, send a message in the ci-request channel in the verl Slack workspace. (If not accessible, please try the Feishu group (飞书群).)

[gemini-code-assist]

Code Review

This pull request enables fused AdamW optimizer and allows setting the epsilon value. The review focuses on ensuring the new parameters are correctly integrated and the fused optimizer is enabled as intended.

[gemini-code-assist]

Enabling fused=True without checking its availability can lead to a crash if the environment doesn't support it. It's critical to verify that the fused kernel is available before enabling it, and provide a fallback if it's not.

Consider wrapping this in a try-except block to catch the import error and switch to a non-fused optimizer if necessary.

        try:
            self.optimizer = optim.AdamW(
                self.fsdp_model.parameters(),
                lr=self.config.optim.lr,
                betas=self.config.optim.betas,
                weight_decay=self.config.optim.weight_decay,
                eps=self.config.optim.get("eps", 1e-08),
                fused=True, # Enable fused AdamW
            )
        except ImportError:
            print("Fused AdamW is not available, falling back to normal AdamW")
            self.optimizer = optim.AdamW(
                self.fsdp_model.parameters(),
                lr=self.config.optim.lr,
                betas=self.config.optim.betas,
                weight_decay=self.config.optim.weight_decay,
                eps=self.config.optim.get("eps", 1e-08),
                fused=False, # Disable fused AdamW
            )

[puneeshkhanna]

@vermouth1992 -FYI. Please review.

[EduardDurech]

@puneeshkhanna after #3656 PR is merged can you make a separate PR for fused kernels as a {actor_rollout_ref.actor,critic}.optim.override_optimizer_config but keep in mind if you hard-code this it won't be compatible with all optimizers, there should be a check