Dont add labels implicitly for parser

spacy/pipeline/dep_parser.pyx

@@ -277,3 +277,10 @@ cdef class DependencyParser(Parser):
             head_scores.append(score_head_dict)
             label_scores.append(score_label_dict)
         return head_scores, label_scores
+
+    def _ensure_labels_are_added(self, docs):
+        # This gives the parser a chance to add labels it's missing for a batch
+        # of documents. However, this isn't desirable for the dependency parser,
+        # because we instead have a label frequency cut-off and back off rare
+        # labels to 'dep'.
+        pass