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Coaching Diffusion Fashions with Reinforcement Studying – The Berkeley Synthetic Intelligence Analysis Weblog



Coaching Diffusion Fashions with Reinforcement Studying

Diffusion fashions have lately emerged because the de facto commonplace for producing advanced, high-dimensional outputs. You could know them for his or her capacity to supply beautiful AI artwork and hyper-realistic artificial photos, however they’ve additionally discovered success in different functions reminiscent of drug design and steady management. The important thing thought behind diffusion fashions is to iteratively rework random noise right into a pattern, reminiscent of a picture or protein construction. That is usually motivated as a most probability estimation downside, the place the mannequin is educated to generate samples that match the coaching knowledge as carefully as potential.

Nevertheless, most use instances of diffusion fashions should not immediately involved with matching the coaching knowledge, however as a substitute with a downstream goal. We don’t simply need a picture that appears like present photos, however one which has a particular sort of look; we don’t simply need a drug molecule that’s bodily believable, however one that’s as efficient as potential. On this publish, we present how diffusion fashions might be educated on these downstream targets immediately utilizing reinforcement studying (RL). To do that, we finetune Steady Diffusion on a wide range of targets, together with picture compressibility, human-perceived aesthetic high quality, and prompt-image alignment. The final of those targets makes use of suggestions from a big vision-language mannequin to enhance the mannequin’s efficiency on uncommon prompts, demonstrating how highly effective AI fashions can be utilized to enhance one another with none people within the loop.

diagram illustrating the RLAIF objective that uses the LLaVA VLM


A diagram illustrating the prompt-image alignment goal. It makes use of LLaVA, a big vision-language mannequin, to judge generated photos.

Denoising Diffusion Coverage Optimization

When turning diffusion into an RL downside, we make solely essentially the most fundamental assumption: given a pattern (e.g. a picture), we’ve entry to a reward perform that we are able to consider to inform us how “good” that pattern is. Our purpose is for the diffusion mannequin to generate samples that maximize this reward perform.

Diffusion fashions are usually educated utilizing a loss perform derived from most probability estimation (MLE), that means they’re inspired to generate samples that make the coaching knowledge look extra possible. Within the RL setting, we not have coaching knowledge, solely samples from the diffusion mannequin and their related rewards. A technique we are able to nonetheless use the identical MLE-motivated loss perform is by treating the samples as coaching knowledge and incorporating the rewards by weighting the loss for every pattern by its reward. This provides us an algorithm that we name reward-weighted regression (RWR), after present algorithms from RL literature.

Nevertheless, there are a number of issues with this strategy. One is that RWR isn’t a very precise algorithm — it maximizes the reward solely roughly (see Nair et. al., Appendix A). The MLE-inspired loss for diffusion can be not precise and is as a substitute derived utilizing a variational sure on the true probability of every pattern. Which means that RWR maximizes the reward via two ranges of approximation, which we discover considerably hurts its efficiency.

chart comparing DDPO with RWR


We consider two variants of DDPO and two variants of RWR on three reward features and discover that DDPO persistently achieves one of the best efficiency.

The important thing perception of our algorithm, which we name denoising diffusion coverage optimization (DDPO), is that we are able to higher maximize the reward of the ultimate pattern if we take note of your entire sequence of denoising steps that received us there. To do that, we reframe the diffusion course of as a multi-step Markov resolution course of (MDP). In MDP terminology: every denoising step is an motion, and the agent solely will get a reward on the ultimate step of every denoising trajectory when the ultimate pattern is produced. This framework permits us to use many highly effective algorithms from RL literature which are designed particularly for multi-step MDPs. As an alternative of utilizing the approximate probability of the ultimate pattern, these algorithms use the precise probability of every denoising step, which is extraordinarily straightforward to compute.

We selected to use coverage gradient algorithms on account of their ease of implementation and previous success in language mannequin finetuning. This led to 2 variants of DDPO: DDPOSF, which makes use of the easy rating perform estimator of the coverage gradient often known as REINFORCE; and DDPOIS, which makes use of a extra highly effective significance sampled estimator. DDPOIS is our best-performing algorithm and its implementation carefully follows that of proximal coverage optimization (PPO).

Finetuning Steady Diffusion Utilizing DDPO

For our fundamental outcomes, we finetune Steady Diffusion v1-4 utilizing DDPOIS. We’ve 4 duties, every outlined by a distinct reward perform:

  • Compressibility: How straightforward is the picture to compress utilizing the JPEG algorithm? The reward is the unfavorable file dimension of the picture (in kB) when saved as a JPEG.
  • Incompressibility: How arduous is the picture to compress utilizing the JPEG algorithm? The reward is the constructive file dimension of the picture (in kB) when saved as a JPEG.
  • Aesthetic High quality: How aesthetically interesting is the picture to the human eye? The reward is the output of the LAION aesthetic predictor, which is a neural community educated on human preferences.
  • Immediate-Picture Alignment: How effectively does the picture signify what was requested for within the immediate? This one is a little more sophisticated: we feed the picture into LLaVA, ask it to explain the picture, after which compute the similarity between that description and the unique immediate utilizing BERTScore.

Since Steady Diffusion is a text-to-image mannequin, we additionally want to choose a set of prompts to present it throughout finetuning. For the primary three duties, we use easy prompts of the shape “a(n) [animal]”. For prompt-image alignment, we use prompts of the shape “a(n) [animal] [activity]”, the place the actions are “washing dishes”, “taking part in chess”, and “driving a motorbike”. We discovered that Steady Diffusion typically struggled to supply photos that matched the immediate for these uncommon eventualities, leaving loads of room for enchancment with RL finetuning.

First, we illustrate the efficiency of DDPO on the easy rewards (compressibility, incompressibility, and aesthetic high quality). All the photos are generated with the identical random seed. Within the high left quadrant, we illustrate what “vanilla” Steady Diffusion generates for 9 totally different animals; the entire RL-finetuned fashions present a transparent qualitative distinction. Apparently, the aesthetic high quality mannequin (high proper) tends in direction of minimalist black-and-white line drawings, revealing the sorts of photos that the LAION aesthetic predictor considers “extra aesthetic”.

results on aesthetic, compressibility, and incompressibility

Subsequent, we reveal DDPO on the extra advanced prompt-image alignment job. Right here, we present a number of snapshots from the coaching course of: every sequence of three photos exhibits samples for a similar immediate and random seed over time, with the primary pattern coming from vanilla Steady Diffusion. Apparently, the mannequin shifts in direction of a extra cartoon-like type, which was not intentional. We hypothesize that it is because animals doing human-like actions usually tend to seem in a cartoon-like type within the pretraining knowledge, so the mannequin shifts in direction of this type to extra simply align with the immediate by leveraging what it already is aware of.

results on prompt-image alignment

Surprising Generalization

Stunning generalization has been discovered to come up when finetuning massive language fashions with RL: for instance, fashions finetuned on instruction-following solely in English typically enhance in different languages. We discover that the identical phenomenon happens with text-to-image diffusion fashions. For instance, our aesthetic high quality mannequin was finetuned utilizing prompts that had been chosen from an inventory of 45 widespread animals. We discover that it generalizes not solely to unseen animals but in addition to on a regular basis objects.

aesthetic quality generalization

Our prompt-image alignment mannequin used the identical record of 45 widespread animals throughout coaching, and solely three actions. We discover that it generalizes not solely to unseen animals but in addition to unseen actions, and even novel mixtures of the 2.

prompt-image alignment generalization

Overoptimization

It’s well-known that finetuning on a reward perform, particularly a realized one, can result in reward overoptimization the place the mannequin exploits the reward perform to attain a excessive reward in a non-useful means. Our setting is not any exception: in all of the duties, the mannequin ultimately destroys any significant picture content material to maximise reward.

overoptimization of reward functions

We additionally found that LLaVA is vulnerable to typographic assaults: when optimizing for alignment with respect to prompts of the shape “[n] animals”, DDPO was capable of efficiently idiot LLaVA by as a substitute producing textual content loosely resembling the proper quantity.

RL exploiting LLaVA on the counting task

There may be presently no general-purpose methodology for stopping overoptimization, and we spotlight this downside as an essential space for future work.

Conclusion

Diffusion fashions are arduous to beat in relation to producing advanced, high-dimensional outputs. Nevertheless, to date they’ve principally been profitable in functions the place the purpose is to study patterns from tons and many knowledge (for instance, image-caption pairs). What we’ve discovered is a approach to successfully practice diffusion fashions in a means that goes past pattern-matching — and with out essentially requiring any coaching knowledge. The probabilities are restricted solely by the standard and creativity of your reward perform.

The way in which we used DDPO on this work is impressed by the latest successes of language mannequin finetuning. OpenAI’s GPT fashions, like Steady Diffusion, are first educated on enormous quantities of Web knowledge; they’re then finetuned with RL to supply helpful instruments like ChatGPT. Usually, their reward perform is realized from human preferences, however others have extra lately discovered methods to produce highly effective chatbots utilizing reward features primarily based on AI suggestions as a substitute. In comparison with the chatbot regime, our experiments are small-scale and restricted in scope. However contemplating the big success of this “pretrain + finetune” paradigm in language modeling, it definitely looks as if it’s value pursuing additional on this planet of diffusion fashions. We hope that others can construct on our work to enhance massive diffusion fashions, not only for text-to-image era, however for a lot of thrilling functions reminiscent of video era, music era,  picture modifying, protein synthesis, robotics, and extra.

Moreover, the “pretrain + finetune” paradigm isn’t the one means to make use of DDPO. So long as you’ve gotten a very good reward perform, there’s nothing stopping you from coaching with RL from the beginning. Whereas this setting is as-yet unexplored, it is a place the place the strengths of DDPO may actually shine. Pure RL has lengthy been utilized to all kinds of domains starting from taking part in video games to robotic manipulation to nuclear fusion to chip design. Including the highly effective expressivity of diffusion fashions to the combo has the potential to take present functions of RL to the following degree — and even to find new ones.


This publish is predicated on the next paper:

If you wish to study extra about DDPO, you may take a look at the paper, web site, authentic code, or get the mannequin weights on Hugging Face. If you wish to use DDPO in your individual undertaking, take a look at my PyTorch + LoRA implementation the place you may finetune Steady Diffusion with lower than 10GB of GPU reminiscence!

If DDPO evokes your work, please cite it with:

@misc{black2023ddpo,
      title={Coaching Diffusion Fashions with Reinforcement Studying}, 
      creator={Kevin Black and Michael Janner and Yilun Du and Ilya Kostrikov and Sergey Levine},
      yr={2023},
      eprint={2305.13301},
      archivePrefix={arXiv},
      primaryClass={cs.LG}
}




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