AttributeError: no attribute 'precision_matrix'

Hi

I just visited your blog and GitHub. It's quite interesting and helpful to me. But somehow when I ran your notebook, I ran into this error. Do you know why? anything missing? (I used the colab and local machine.)

```python
AttributeError: type object 'MoG2' has no attribute 'precision_matrix'
```

Thanks

Reproduce the issue by opening and running the notebook that uses `MoG2` until the `AttributeError` is triggered. Find where `MoG2` is defined in the repo and confirm whether `precision_matrix` should be defined as a class attribute, instance attribute, or comes from a dependency. Compare that expected API against the notebook usage and mark done when the notebook path executes without raising `AttributeError` for `precision_matrix`.

由索引模型根据 Issue 内容生成。