Add library_name and clean up model card

This PR improves the model card by:
- Adding `library_name: diffusers` to the metadata, which enables the "How to use" widget on the model page with a `diffusers` code snippet.
- Removing a redundant YAML frontmatter block from the markdown content, as all metadata is already (or now will be) correctly placed in the top-level YAML block.

README.md

@@ -1,15 +1,17 @@
 ---
-license: apache-2.0
-datasets:
-- APRIL-AIGC/UltraVideo
 base_model:
 - Wan-AI/Wan2.1-T2V-1.3B
+datasets:
+- APRIL-AIGC/UltraVideo
+license: apache-2.0
 pipeline_tag: text-to-video
 tags:
 - video
 - t2v
 - i2v
+library_name: diffusers
 ---
+
 # T3-Video
 
 -----