<img src="./fig3.png" width="400px"></img>

## pi-zero-pytorch (wip)

Implementation of <a href="https://www.physicalintelligence.company/blog/pi0">π₀</a> the robotic foundation model architecture proposed by Physical Intelligence

Summary of this work would be that it is a simplified <a href="https://github.com/lucidrains/transfusion-pytorch">Transfusion</a> (Zhou et al.) with influence from <a href="https://arxiv.org/abs/2403.03206">Stable Diffusion 3</a> (Esser et al.), mainly the adoption of flow matching instead of diffusion for policy generation, as well as the separation of parameters (<a href="https://github.com/lucidrains/mmdit/blob/main/mmdit/mmdit_pytorch.py#L43">Joint Attention</a> from mmDIT). They build on top of a pretrained vision language model, PaliGemma 2B.

Update: The [official repository](https://github.com/Physical-Intelligence/openpi) has been open sourced!

### Appreciation

- [Einops](https://github.com/arogozhnikov/einops) for the amazing [pack and unpack](https://einops.rocks/4-pack-and-unpack/), used extensively here for managing various token sets

- [Flex Attention](https://pytorch.org/blog/flexattention/) for allowing for easy mixture of autoregressive and bidirectional attention

- [@Wonder1905](https://github.com/Wonder1905) for the code review and identifying issues

- You? maybe a phd student who wants to contribute to the latest SOTA architecture for behavioral cloning?

### Install

```bash
$ pip install pi-zero-pytorch
```

### Usage

```python
import torch
from pi_zero_pytorch import π0

model = π0(
    dim = 512,
    dim_action_input = 6,
    dim_joint_state = 12,
    num_tokens = 20_000
)

vision = torch.randn(1, 1024, 512)
commands = torch.randint(0, 20_000, (1, 1024))
joint_state = torch.randn(1, 12)
actions = torch.randn(1, 32, 6)

loss, _ = model(vision, commands, joint_state, actions)
loss.backward()

# after much training

sampled_actions = model(vision, commands, joint_state, trajectory_length = 32) # (1, 32, 6)
```

To do online learning, just wrap the model with the `Agent` class

```python
from pi_zero_pytorch import π0, Agent, EPO

# wrap the model with `Agent`, which will instantiate actor and critic for PPO

agent = Agent(model)

# you'll want to supply your own environment

from pi_zero_pytorch.mock_env import Env
mock_env = Env((256, 256), 2, 32, 1024, 12)

# pass your agent and environment to EPO for learning to be orchestrated

epo = EPO(agent, mock_env)

# gather memories from environment

memories = epo.gather_experience_from_env(steps = 10)

# learn from memories

epo.learn_agent(memories, batch_size = 2)
```

### Contributing

At the project root, run

```bash
$ pip install '.[test]' # or `uv pip install '.[test]'`
```

Then add your tests to `tests/test_pi_zero.py` and run

```bash
$ pytest tests/
```

That's it

### Citation

```bibtex
@misc{Black2024,
    author  = {Kevin Black, Noah Brown, Danny Driess, Adnan Esmail, Michael Equi, Chelsea Finn, Niccolo Fusai, Lachy Groom, Karol Hausman, Brian Ichter, Szymon Jakubczak, Tim Jones, Liyiming Ke, Sergey Levine, Adrian Li-Bell, Mohith Mothukuri, Suraj Nair, Karl Pertsch, Lucy Xiaoyang Shi, James Tanner, Quan Vuong, Anna Walling, Haohuan Wang, Ury Zhilinsky},
    url     = {https://www.physicalintelligence.company/download/pi0.pdf}
}
```

```bibtex
@inproceedings{Zhou2024ValueRL,
    title   = {Value Residual Learning For Alleviating Attention Concentration In Transformers},
    author  = {Zhanchao Zhou and Tianyi Wu and Zhiyun Jiang and Zhenzhong Lan},
    year    = {2024},
    url     = {https://api.semanticscholar.org/CorpusID:273532030}
}
```

```bibtex
@inproceedings{Darcet2023VisionTN,
    title   = {Vision Transformers Need Registers},
    author  = {Timoth'ee Darcet and Maxime Oquab and Julien Mairal and Piotr Bojanowski},
    year    = {2023},
    url     = {https://api.semanticscholar.org/CorpusID:263134283}
}
```

```bibtex
@article{Li2024ImmiscibleDA,
    title   = {Immiscible Diffusion: Accelerating Diffusion Training with Noise Assignment},
    author  = {Yiheng Li and Heyang Jiang and Akio Kodaira and Masayoshi Tomizuka and Kurt Keutzer and Chenfeng Xu},
    journal = {ArXiv},
    year    = {2024},
    volume  = {abs/2406.12303},
    url     = {https://api.semanticscholar.org/CorpusID:270562607}
}
```

```bibtex
@inproceedings{Sadat2024EliminatingOA,
    title   = {Eliminating Oversaturation and Artifacts of High Guidance Scales in Diffusion Models},
    author  = {Seyedmorteza Sadat and Otmar Hilliges and Romann M. Weber},
    year    = {2024},
    url     = {https://api.semanticscholar.org/CorpusID:273098845}
}
```

```bibtex
@article{Bulatov2022RecurrentMT,
    title   = {Recurrent Memory Transformer},
    author  = {Aydar Bulatov and Yuri Kuratov and Mikhail S. Burtsev},
    journal = {ArXiv},
    year    = {2022},
    volume  = {abs/2207.06881},
    url     = {https://api.semanticscholar.org/CorpusID:250526424}
}
```

```bibtex
@inproceedings{Bessonov2023RecurrentAT,
    title   = {Recurrent Action Transformer with Memory},
    author  = {A. B. Bessonov and Alexey Staroverov and Huzhenyu Zhang and Alexey K. Kovalev and D. Yudin and Aleksandr I. Panov},
    year    = {2023},
    url     = {https://api.semanticscholar.org/CorpusID:259188030}
}
```

```bibtex
@article{Zhu2024HyperConnections,
    title   = {Hyper-Connections},
    author  = {Defa Zhu and Hongzhi Huang and Zihao Huang and Yutao Zeng and Yunyao Mao and Banggu Wu and Qiyang Min and Xun Zhou},
    journal = {ArXiv},
    year    = {2024},
    volume  = {abs/2409.19606},
    url     = {https://api.semanticscholar.org/CorpusID:272987528}
}
```

```bibtex
@inproceedings{Sun2025F5RTTSIF,
    title   = {F5R-TTS: Improving Flow-Matching based Text-to-Speech with Group Relative Policy Optimization},
    author  = {Xiaohui Sun and Ruitong Xiao and Jianye Mo and Bowen Wu and Qun Yu and Baoxun Wang},
    year    = {2025},
    url     = {https://api.semanticscholar.org/CorpusID:277510064}
}
```

```bibtex
@inproceedings{Wang2025EvolutionaryPO,
    title = {Evolutionary Policy Optimization},
    author = {Jianren Wang and Yifan Su and Abhinav Gupta and Deepak Pathak},
    year  = {2025},
    url   = {https://api.semanticscholar.org/CorpusID:277313729}
}
```

```bibtex
@misc{PI2025,
    title   = {Real-Time Action Chunking with Large Models},
    author  = {Kevin Black, Manuel Y. Galliker, Sergey Levine},
    year    = {2025},
    url     = {https://www.pi.website/research/real_time_chunking}
}
```

```bibtex
@misc{PI2025,
    title = {VLAs that Train Fast, Run Fast, and Generalize Better},
    author = {Danny Driess, Jost Tobias Springenberg, Brian Ichter, Lili Yu, Adrian Li-Bell, Karl Pertsch, Allen Z. Ren, Homer Walke, Quan Vuong, Lucy Xiaoyang Shi, Sergey Levine},
    year   = {2025},
    url    = {https://www.physicalintelligence.company/research/knowledge_insulation}
}
```

```bibtex
@inproceedings{Wagenmaker2025SteeringYD,
    title   = {Steering Your Diffusion Policy with Latent Space Reinforcement Learning},
    author  = {Andrew Wagenmaker and Mitsuhiko Nakamoto and Yunchu Zhang and Seohong Park and Waleed Yagoub and Anusha Nagabandi and Abhishek Gupta and Sergey Levine},
    year    = {2025},
    url     = {https://api.semanticscholar.org/CorpusID:279464702}
}
```

```bibtex
@misc{dong2025reinforcementlearningimplicitimitation,
    title   = {Reinforcement Learning via Implicit Imitation Guidance}, 
    author  = {Perry Dong and Alec M. Lessing and Annie S. Chen and Chelsea Finn},
    year    = {2025},
    eprint  = {2506.07505},
    archivePrefix = {arXiv},
    primaryClass = {cs.LG},
    url = {https://arxiv.org/abs/2506.07505}, 
}
```

```bibtex
@misc{zhou2025efficientonlinereinforcementlearning,
    title   = {Efficient Online Reinforcement Learning Fine-Tuning Need Not Retain Offline Data}, 
    author  = {Zhiyuan Zhou and Andy Peng and Qiyang Li and Sergey Levine and Aviral Kumar},
    year    = {2025},
    eprint  = {2412.07762},
    archivePrefix = {arXiv},
    primaryClass = {cs.LG},
    url = {https://arxiv.org/abs/2412.07762}, 
}
```

```bibtex
@misc{cheang2025gr3technicalreport,
    title   = {GR-3 Technical Report}, 
    author  = {Chilam Cheang and Sijin Chen and Zhongren Cui and Yingdong Hu and Liqun Huang and Tao Kong and Hang Li and Yifeng Li and Yuxiao Liu and Xiao Ma and Hao Niu and Wenxuan Ou and Wanli Peng and Zeyu Ren and Haixin Shi and Jiawen Tian and Hongtao Wu and Xin Xiao and Yuyang Xiao and Jiafeng Xu and Yichu Yang},
    year    = {2025},
    eprint  = {2507.15493},
    archivePrefix = {arXiv},
    primaryClass = {cs.RO},
    url     = {https://arxiv.org/abs/2507.15493}, 
}
```

```bibtex
@misc{heng2025vitacformerlearningcrossmodalrepresentation,
    title   = {ViTacFormer: Learning Cross-Modal Representation for Visuo-Tactile Dexterous Manipulation}, 
    author  = {Liang Heng and Haoran Geng and Kaifeng Zhang and Pieter Abbeel and Jitendra Malik},
    year    = {2025},
    eprint  = {2506.15953},
    archivePrefix = {arXiv},
    primaryClass = {cs.RO},
    url     = {https://arxiv.org/abs/2506.15953}, 
}
```

```bibtex
@misc{mcallister2025flowmatchingpolicygradients,
    title   = {Flow Matching Policy Gradients}, 
    author  = {David McAllister and Songwei Ge and Brent Yi and Chung Min Kim and Ethan Weber and Hongsuk Choi and Haiwen Feng and Angjoo Kanazawa},
    year    = {2025},
    eprint  = {2507.21053},
    archivePrefix = {arXiv},
    primaryClass = {cs.LG},
    url = {https://arxiv.org/abs/2507.21053}, 
}
```

[*dear alice*](https://www.youtube.com/watch?v=z-Ng5ZvrDm4)
