[Project] add x-decoder link (#3142)

projects/XDecoder/README.md

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+# X-Decoder
+
+> [X-Decoder: Generalized Decoding for Pixel, Image, and Language](https://arxiv.org/pdf/2212.11270.pdf)
+
+<!-- [ALGORITHM] -->
+
+## Abstract
+
+We present X-Decoder, a generalized decoding model that can predict pixel-level segmentation and language tokens seamlessly. X-Decodert takes as input two types of queries: (i) generic non-semantic queries and (ii) semantic queries induced from text inputs, to decode different pixel-level and token-level outputs in the same semantic space. With such a novel design, X-Decoder is the first work that provides a unified way to support all types of image segmentation and a variety of vision-language (VL) tasks. Further, our design enables seamless interactions across tasks at different granularities and brings mutual benefits by learning a common and rich pixel-level visual-semantic understanding space, without any pseudo-labeling. After pretraining on a mixed set of a limited amount of segmentation data and millions of image-text pairs, X-Decoder exhibits strong transferability to a wide range of downstream tasks in both zero-shot and finetuning settings. Notably, it achieves (1) state-of-the-art results on open-vocabulary segmentation and referring segmentation on eight datasets; (2) better or competitive finetuned performance to other generalist and specialist models on segmentation and VL tasks; and (3) flexibility for efficient finetuning and novel task composition (e.g., referring captioning and image editing).
+
+<div align=center>
+<img src="https://github.com/open-mmlab/mmdetection/assets/17425982/cb126615-9402-4c19-8ea9-133722d7519c" width="70%"/>
+</div>
+
+## Usage
+
+We implement it based on [mmdetection](https://github.com/open-mmlab/mmdetection/), please refer to [mmdetection/projects/XDecoder](https://github.com/open-mmlab/mmdetection/tree/main/projects/XDecoder) for more details.