is Default stream existed? #71

I created two stream with NON_NLOCKING flag as below, to test host2dev memory copy and kernal run concurrency. 
unfortunately, They seems still execute in order. if there is default stream stay in system or the concurrency not support now? thanks!
 let stream_0 = Stream::new(StreamFlags::NON_BLOCKING, None)?;
 let stream_1 = Stream::new(StreamFlags::NON_BLOCKING, None)?;