[Chapter 2] Exercise 5

What means this `'feature_selection__k': list(range(1, len(feature_importances) + 1))` code in:

```
param_grid = [{
'preparation__num__imputer__strategy': ['mean', 'median', 'most_frequent'],
'feature_selection__k': list(range(1, len(feature_importances) + 1))
}]

grid_search_prep = GridSearchCV(prepare_select_and_predict_pipeline, param_grid, cv=5,
scoring='neg_mean_squared_error', verbose=2)
grid_search_prep.fit(housing, housing_labels)
```