EHN vectorize true-class probability calculation of InstanceHardnessThreshold (#566)

Author: Shihab-Shahriar

imblearn/under_sampling/_prototype_selection/_instance_hardness_threshold.py

@@ -157,11 +157,7 @@ def _fit_resample(self, X, y):
             self.estimator_.fit(X_train, y_train)
 
             probs = self.estimator_.predict_proba(X_test)
-            classes = self.estimator_.classes_
-            probabilities[test_index] = [
-                probs[l, np.where(classes == c)[0][0]]
-                for l, c in enumerate(y_test)
-            ]
+            probabilities[test_index] = probs[range(len(y_test)), y_test]
 
         idx_under = np.empty((0, ), dtype=int)