add warning to remind Optimizer() deprecates (#3062)
add warning to remind Optimizer() deprecates (#3062)

* add warning to remind deprecates
yangw1234/BigDL,yangw1234/BigDL,yangw1234/BigDL,intel-analytics/BigDL,intel-analytics/BigDL,intel-analytics/BigDL,yangw1234/BigDL,intel-analytics/BigDL