TFOptimizer support sparse gradient (#987)
TFOptimizer support sparse gradient (#987)

* TFOptimizer support sparse gradient

* fix test

* fix test

* fix test

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