说明：  feather_stacker主要用于一个数据集提取多个特征时，对特征进行联合的算法。
(In many real-world examples, there are many ways to extract features a dataset. Often it is beneficial to combine several methods to obtain good performance. This example shows how to use FeatureUnion to combine features obtained by PCA and univariate selection. Combining features using this transformer has the benefit that it allows cross validation and grid searches over the whole process. The combination used in this example is not particularly helpful on this dataset and is only used to illustrate the usage of FeatureUnion.)