ByteDance-Seed/BAGEL-7B-MoT · Improve model card: Update `library_name`, add `multimodal` tag, and add sample usage

This PR improves the model card for the BAGEL model by:

Updating the library_name metadata tag from bagel-mot to transformers, aligning it with the model's actual compatibility and enhancing discoverability on the Hugging Face Hub.
Adding the multimodal tag to the metadata, which accurately reflects the model's capabilities in unified multimodal understanding and generation.
Integrating a comprehensive sample usage section with Python code snippets directly into the model card, demonstrating how to use the model for both text-only and multimodal (text and image) inputs via the transformers library.

These changes make the model card more informative, discoverable, and user-friendly.