!7731 Bind the fvs with single fg bprop for switch layer.

Merge pull request !7731 from 张清华/grad_opt

mindspore/ccsrc/frontend/optimizer/ad/dfunctor.cc

@@ -130,6 +130,7 @@ void DFunctor::BackPropagateSwitchLayer(const CNodePtr &cnode_morph, const CNode
   if (!IsPrimitiveCNode(input, prim::kPrimMakeTuple)) {
     MS_LOG(EXCEPTION) << "The 2th input of switch_layer expect a tuple of graphs, but got " << input->ToString() << ".";
   }
+  std::unordered_map<AnfNodePtr, FuncGraphPtr> node_to_fg;
   auto tuple_graphs = input->cast<CNodePtr>();
   for (size_t i = 1; i < tuple_graphs->size(); ++i) {
     auto graph = tuple_graphs->input(i);
@@ -145,11 +146,19 @@ void DFunctor::BackPropagateSwitchLayer(const CNodePtr &cnode_morph, const CNode
     }
     // Consider direct and indirect fvs.
     for (auto fv : func_graph->free_variables_nodes()) {
+      if (node_to_fg.find(fv) != node_to_fg.end()) {
+        continue;
+      }
+      node_to_fg[fv] = func_graph;
       BackPropagateFv(fv, env);
     }
     for (auto indirect_fv : functor->second->anfnode_to_adjoin_indirect_fv_) {
       MS_LOG(DEBUG) << "BackPropagateSwitchLayer backprop indirect fv " << func_graph->ToString() << " "
                     << indirect_fv.first->ToString() << ".";
+      if (node_to_fg.find(indirect_fv.first) != node_to_fg.end()) {
+        continue;
+      }
+      node_to_fg[indirect_fv.first] = func_graph;
       BackPropagateFv(indirect_fv.first, env);
     }
   }