The notebook example is not working

Nadie ha tomado este issue todavía.

I tried to run the notebook example with the following code snippet:

```
shap_explainer = fasttreeshap.TreeExplainer(lgb_model)
num_leaves = sum(shap_explainer.model.num_nodes) - sum(sum(shap_explainer.model.children_left > 0))
print("Total number of leaves is {}.".format(num_leaves))
```
But got the error:
```
AttributeError Traceback (most recent call last)
in ()
1 shap_explainer = fasttreeshap.TreeExplainer(lgb_model)
----> 2 num_leaves = sum(shap_explainer.model.num_nodes) - sum(sum(shap_explainer.model.children_left > 0))
3 print("Total number of leaves is {}.".format(num_leaves))

AttributeError: 'TreeEnsemble' object has no attribute 'num_nodes'
```
The lightbgm library version is '3.3.5'. Maybe you want to update the notebook correspondingly.

Localiza el ejemplo de notebook que contiene el fragmento mostrado de TreeExplainer y num_leaves, y reproduce el AttributeError con LightGBM 3.3.5. Actualiza el ejemplo para que se ejecute con esa versión y verifica que el notebook se complete mientras informa del número total de hojas.

Escrito por el modelo de indexación a partir del texto del issue.