Add library_name and code highlighting

This PR ensures a "Use this model" button appears at the top right along with code highlighting.

README.md

@@ -1,4 +1,5 @@
 ---
+library_name: transformers
 license: unknown
 base_model:
 - Qwen/Qwen2.5-Coder-7B
@@ -22,7 +23,7 @@ The DiffuCoder-7B-Base model is our foundational masked diffusion LLM for code g
 
 - GitHub: https://github.com/apple/ml-diffucoder
 
-```
+```python
 import torch
 from transformers import AutoModel, AutoTokenizer