Support feed prior prediction back into recurrent (#1476)
Support feed prior prediction back into recurrent (#1476)

* Implement RecurrentDecoder
* add ut for lstm with recurrent decoder
yangw1234/BigDL,intel-analytics/BigDL,yangw1234/BigDL,yangw1234/BigDL,intel-analytics/BigDL,intel-analytics/BigDL,yangw1234/BigDL,intel-analytics/BigDL