[Feature] Refactor weight loader for deepseek_v2.py

### Checklist

- [ ] If this is not a feature request but a general question, please start a discussion at https://github.com/sgl-project/sglang/discussions. Otherwise, it will be closed.
- [ ] Please use English. Otherwise, it will be closed.

### Motivation

Part of #16255
cc @xyjixyjixyji

### Related resources

_No response_