Fixes bug when using LCHT with an Evaluator.

- Override partial_fit to infer classes by default.

src/skmultiflow/trees/lc_hoeffding_tree.py

@@ -104,6 +104,9 @@ def n_labels(self, n_labels):
             raise ValueError('The number of labels must be specified')
         self._n_labels = n_labels
 
+    def partial_fit(self, X, y, classes=None, weight=None):
+        super().partial_fit(X, y, weight=weight)    # Override HT, infer the classes
+
     class LCActiveLearningNode(HoeffdingTree.ActiveLearningNode):
 
         def __init__(self, initial_class_observations):