Optimize recurrent_op using Prepare and RunPreparedContext, avoiding create operators in every iter. (#17689)

test=develop

paddle/fluid/operators/recurrent_op.cc

@@ -272,6 +272,9 @@ class RecurrentOp : public RecurrentBase {
     auto *block = Attr<framework::BlockDesc *>(kStepBlock);
 
     auto *program = block->Program();
+    auto ctx = executor.Prepare(
+        *program, block->ID(), std::vector<std::string>() /*skip_ref_cnt_vars*/,
+        true /*force_disable_gc*/);
 
     for (size_t i = 0; i < seq_len; ++i) {
       size_t seq_offset = reverse ? seq_len - i - 1 : i;
@@ -305,10 +308,9 @@ class RecurrentOp : public RecurrentBase {
       }
 
       // Every inputs are linked now, execute!
-      executor.Run(*program, &cur_scope, block->ID(),
-                   false /*create_local_scope*/, true /*create_vars*/,
-                   std::vector<std::string>() /*skip_ref_cnt_vars*/,
-                   true /*force_disable_gc*/);
+      executor.RunPreparedContext(ctx.get(), &cur_scope,
+                                  false /*create_local_scope*/,
+                                  true /*create_vars*/, true /* keep_kids */);
 
       // Copy inside::output -> outside::output
       //    outside::output[seq_offset: seq_offset + 1] = inside::output
@@ -366,6 +368,9 @@ class RecurrentGradOp : public RecurrentBase {
     framework::Executor executor(place);
     auto *block = Attr<framework::BlockDesc *>(kStepBlock);
     auto *program = block->Program();
+    auto ctx = executor.Prepare(
+        *program, block->ID(), std::vector<std::string>() /*skip_ref_cnt_vars*/,
+        true /*force_disable_gc*/);
 
     for (size_t step_id = 0; step_id < seq_len; ++step_id) {
       size_t seq_offset = reverse ? step_id : seq_len - step_id - 1;
@@ -423,10 +428,9 @@ class RecurrentGradOp : public RecurrentBase {
 
       VLOG(5) << "Recurrent memory linking finished ";
       // Run step block with cur_scope
-      executor.Run(*program, &cur_scope, block->ID(),
-                   false /*create_local_scope*/, true /*create_vars*/,
-                   std::vector<std::string>() /*skip_ref_cnt_vars*/,
-                   true /*force_disable_gc*/);
+      executor.RunPreparedContext(ctx.get(), &cur_scope,
+                                  false /*create_local_scope*/,
+                                  true /*create_vars*/, true /* keep_kids */);
 
       VLOG(5) << "executor.Run finished ";