This PR improves the model card for DiffGap by:
- Adding the `pipeline_tag: other` metadata.
- Linking the model to the associated research paper: [Bridging the Gap between Learning and Inference for Diffusion-Based Molecule Generation](https://huggingface.co/papers/2411.05472).
- Adding a link to the official GitHub repository.
- Providing sample usage instructions for training, sampling, and evaluation based on the official implementation.
- Including the BibTeX citation for the paper.
