add Exponential learning rate decay (#1103)
add Exponential learning rate decay (#1103)

* add Exponential learning rate decay

* fix unit test

yangw1234/BigDL,intel-analytics/BigDL,intel-analytics/BigDL,yangw1234/BigDL,yangw1234/BigDL,yangw1234/BigDL,intel-analytics/BigDL,intel-analytics/BigDL