add comment that out var of prefetch must be created in local scope

Author: jacquesqiao

paddle/fluid/operators/detail/grpc_server.cc

@@ -169,6 +169,7 @@ class RequestPrefetch final : public RequestBase {
 
     auto scope = request_->GetMutableLocalScope();
     auto invar = scope->FindVar(in_var_name);
+    // out var must be created in local scope!
     framework::Variable* outvar = scope->Var(out_var_name);
 
     request_handler_->Handle(in_var_name, scope, invar, &outvar, out_var_name);