add nodes for drnn

test=develop

paddle/fluid/operators/while_op.cc

@@ -224,10 +224,12 @@ class WhileGradOp : public framework::OperatorBase {
         if (cur_scope_iter == step_scopes->rbegin()) {
           auto *var = (*cur_scope_iter)->FindVar(inside_grad_name);
           PADDLE_ENFORCE_NOT_NULL(var, "Can not find var %s", inside_grad_name);
-          PADDLE_ENFORCE(var->IsType<framework::LoDTensorArray>() ||
+          PADDLE_ENFORCE(
+              var->IsType<framework::LoDTensorArray>() ||
                   var->IsType<LoDTensor>(),
-                         "Currently the type of var only can be LoDTensorArray "
-                         "or LoDTensor.");
+              "Currently the type of var only can be LoDTensorArray, "
+              "or LoDTensor, but the received var[%s] is %s.",
+              inside_grad_name, var->Type().name());
 
           if (var->IsType<LoDTensor>()) {
             auto &inside_tensor = var->Get<framework::LoDTensor>();

python/paddle/fluid/layers/control_flow.py

@@ -1570,6 +1570,10 @@ class DynamicRNN(object):
 
     The dynamic RNN can mark multiple variables as its output. Use `drnn()` to
     get the output sequence.
+    
+    NOTES:
+        Currently it is not supported that setting is_sparse to True of any 
+        layers within DynamicRNN.
     """
     BEFORE_RNN = 0
     IN_RNN = 1