Remove nonzero occurence in IHT

Author: Guillaume Lemaitre

imblearn/under_sampling/instance_hardness_threshold.py

@@ -193,7 +193,7 @@ def _sample(self, X, y):
 
         # If we need to offer support for the indices
         if self.return_indices:
-            idx_under = np.nonzero(mask)[0]
+            idx_under = np.flatnonzero(mask)
             return X_resampled, y_resampled, idx_under
         else:
             return X_resampled, y_resampled