title: What does rank_test_score stand for from the model.cv_results_?
body: <p>After I have built a model with GridSearchCV, I get the cross validation results with model.cv_results_. But among the results one parameter is confusing to me. What does <strong>rank_test_score</strong> stand for in this?</p>
  
  <pre><code>mean_fit_time                                        0.00265972
  std_fit_time                                        0.000466648
  mean_score_time                                      0.00133236
  std_score_time                                      0.000470977
  param_n_neighbors                                             1
  param_weights                                          distance
  params                {'n_neighbors': 1, 'weights': 'distance'}
  split0_test_score                                       0.70405
  split1_test_score                                       0.73125
  split2_test_score                                       0.69906
  mean_test_score                                        0.711458
  std_test_score                                        0.0141423
  rank_test_score                                               1
  split0_train_score                                            1
  split1_train_score                                            1
  split2_train_score                                            1
  mean_train_score                                              1
  std_train_score                                               0
  Name: 1, dtype: object
  </code></pre>