Add model card and metadata

This PR adds a model card for M2Retinexformer, including:
- Metadata with the `image-to-image` pipeline tag.
- A link to the paper: [M2Retinexformer: Multi-Modal Retinexformer for Low-Light Image Enhancement](https://huggingface.co/papers/2605.12556).
- A link to the official GitHub repository.
- A summary of the model's architecture and the BibTeX citation.

README.md

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+---
+pipeline_tag: image-to-image
+---
+
+# M2Retinexformer
+
+This repository contains the official weights for **M2Retinexformer** (Multi-Modal Retinexformer), introduced in the paper [M2Retinexformer: Multi-Modal Retinexformer for Low-Light Image Enhancement](https://huggingface.co/papers/2605.12556).
+
+- **Paper:** [M2Retinexformer: Multi-Modal Retinexformer for Low-Light Image Enhancement](https://huggingface.co/papers/2605.12556)
+- **Code:** [GitHub Repository](https://github.com/YoussefAboelwafa/M2Retinexformer)
+
+## Introduction
+
+Low-light image enhancement is challenging due to complex degradations, including amplified noise, artifacts, and color distortion. M2Retinexformer is a novel framework that extends [Retinexformer](https://arxiv.org/abs/2303.06705) by incorporating **depth cues**, **luminance priors**, and **semantic features** within a progressive refinement pipeline. 
+
+Depth provides geometric context invariant to lighting variations, while luminance and semantic features offer explicit guidance on brightness distribution and scene understanding. These modalities are fused through cross-attention with adaptive gating to dynamically balance illumination-guided self-attention and cross-attention based on the reliability of auxiliary cues.
+
+## Citation
+
+If you find this work useful, please cite:
+
+```bibtex
+@misc{aboelwafa2026m2retinexformermultimodalretinexformerlowlight,
+      title={M2Retinexformer: Multi-Modal Retinexformer for Low-Light Image Enhancement}, 
+      author={Youssef Aboelwafa and Hicham G. Elmongui and Marwan Torki},
+      year={2026},
+      eprint={2605.12556},
+      archivePrefix={arXiv},
+      primaryClass={cs.CV},
+      url={https://arxiv.org/abs/2605.12556}, 
+}
+```
+
+## Acknowledgements
+
+This project is built on the baseline architecture of [Retinexformer](https://github.com/caiyuanhao1998/Retinexformer).