docstr: Fit the training data to FeatureSelector
  
  Returns
  -------
  list :
          Return the index of imprtant feature.
docstr_tok: ["Fit","the","training","data","to","FeatureSelector","Returns","--","--","--","-","list",":","Return","the","index","of","imprtant","feature","."]
code: def get_selected_features(self, topk):
      """
      Fit the training data to FeatureSelector
  
      Returns
      -------
      list :
              Return the index of imprtant feature.
      """
      assert topk > 0
  
      self.selected_features_ = self.feature_importance.argsort()[
          -topk:
      ][::-1]
  
      return self.selected_features_
  
file_path: nni/nni/algorithms/feature_engineering/gbdt_selector/gbdt_selector.py
imports: import random
  from sklearn.model_selection import train_test_split
  import lightgbm
  from nni.feature_engineering.feature_selector import FeatureSelector
label_desc: Use image node_id 3 for calling the GBDTSelector obj's underlying member method code with example usage: obj.get_selected_features(topk) and returns: self