Fix ut

Author: 继盛

mars/services/task/analyzer/assigner.py

@@ -164,8 +164,7 @@ def _assign_by_bfs(
     def _build_undirected_chunk_graph(
         self, chunk_to_assign: List[ChunkData]
     ) -> ChunkGraph:
-        chunk_graph = ChunkGraph()
-        self._chunk_graph.copyto(chunk_graph)
+        chunk_graph = self._chunk_graph.copy()
         # remove edges for all chunk_to_assign which may contain chunks
         # that need be reassigned
         for chunk in chunk_to_assign: