[Cherry-pick]Fix the double grad bug for the star gan. (#25655) (#25964)

* Fix the double grad bug for the star gan. (#25655)

* update the retain_graph parameter doc. test=develop

paddle/fluid/imperative/basic_engine.cc

@@ -33,8 +33,10 @@
 namespace paddle {
 namespace imperative {
 
-void BasicEngine::Init(VarBase* var, const detail::BackwardStrategy& strategy) {
+void BasicEngine::Init(VarBase* var, const detail::BackwardStrategy& strategy,
+                       bool retain_graph) {
   backward_strategy_ = strategy;
+  retain_graph_ = retain_graph;
   init_node_ = var->GradVarBase()->GradNode();
   var->GradVarBase()->ClearGradNode();
 
@@ -224,7 +226,9 @@ void BasicEngine::Execute() {
       need_accu_var_list_.clear();
 
       VLOG(3) << "Remove op after op " << cur_op.Type() << " runs";
-      cur_op.ClearBackwardTrace();
+      if (!retain_graph_) {
+        cur_op.ClearBackwardTrace();
+      }
     }
 
     // Step 3: Collect ready ops

paddle/fluid/imperative/basic_engine.h

@@ -30,7 +30,8 @@ class OpBase;
 
 class BasicEngine : public Engine {
  public:
-  void Init(VarBase* var, const detail::BackwardStrategy& strategy);
+  void Init(VarBase* var, const detail::BackwardStrategy& strategy,
+            bool retain_graph = false);
 
   void Execute() override;
 
@@ -51,6 +52,7 @@ class BasicEngine : public Engine {
       accumulators_;
   std::vector<std::pair<GradientAccumulator*, std::shared_ptr<VariableWrapper>>>
       need_accu_var_list_;
+  bool retain_graph_;
 };
 
 }  // namespace imperative

paddle/fluid/pybind/imperative.cc

@@ -694,11 +694,11 @@ void BindImperative(py::module *m_ptr) {
       .def("_run_backward",
            [](imperative::VarBase &self,
               const imperative::detail::BackwardStrategy &bckst,
-              const imperative::Tracer &tracer) {
+              const imperative::Tracer &tracer, bool retain_graph) {
              // TODO(jiabin): when we impl more backward execution we can
              // select them
              auto *engine = tracer.GetEngine();
-             engine->Init(&self, bckst);
+             engine->Init(&self, bckst, retain_graph);
              VLOG(3) << "Start backward";
              engine->Execute();
              VLOG(3) << "Finish backward";

python/paddle/fluid/dygraph/varbase_patch_methods.py

@@ -73,7 +73,7 @@ def monkey_patch_varbase():
                                       framework._current_expected_place())
 
     @framework.dygraph_only
-    def backward(self, backward_strategy=None):
+    def backward(self, backward_strategy=None, retain_graph=False):
         """
         **Notes**:
             **This API is ONLY available in Dygraph mode**
@@ -82,6 +82,10 @@ def monkey_patch_varbase():
 
         Args:
             backward_strategy( :ref:`api_fluid_dygraph_BackwardStrategy` ): The Backward Strategy to run backward
+            retain_graph(bool, optional): If False, the graph used to compute grads will be freed. If you would
+            like to add more ops to the built graph after calling this method(`backward`), set the parameter
+            `retain_graph` to True, then the grads will be retained. Thus, seting it to False is much more memory-efficient.
+            Defaults to False.
 
         Returns:
             NoneType: None
@@ -113,7 +117,8 @@ def monkey_patch_varbase():
                 backward_strategy = BackwardStrategy()
                 backward_strategy.sort_sum_gradient = False
 
-            self._run_backward(backward_strategy, framework._dygraph_tracer())
+            self._run_backward(backward_strategy,
+                               framework._dygraph_tracer(), retain_graph)
         else:
             raise ValueError(
                 "Variable.backward() is only available in DyGraph mode")