[TUTORIAL] Fix vta tutorial after relay function refactor (#5095)

vta/python/vta/top/graphpack.py

@@ -368,8 +368,8 @@ def get_subgraph(expr, start_name, stop_name, start_name_idx, stop_name_idx, cou
     def _recursion(anf, start_found, stop_found, operator_current_idx):
         """ Helper to obtain the subgraph.
         """
-        if isinstance(anf, relay.expr.Function):
-            return relay.expr.Function(anf.params,
+        if isinstance(anf, relay.Function):
+            return relay.Function(anf.params,
                                   _recursion(anf.body, start_found, stop_found,
                                              operator_current_idx),
                                   anf.ret_type, anf.type_params, anf.attrs)