Add dataset card

Adds a dataset card to the repository.

README.md

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-license: mit
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+---
+license: mit
+task_categories:
+- image-to-image
+---
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+Dataset for [Step1X-Edit: A Practical Framework for General Image Editing](https://arxiv.org/abs/2504.17761).
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+This dataset is a new benchmark, grounded in real-world usages is developed to support more authentic and comprehensive evaluation of image editing models.
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+[Code](https://github.com/stepfun-ai/Step1X-Edit)