Keras support - add Hard-sigmoid layer (#1897)
Keras support - add Hard-sigmoid layer (#1897)

* add hard sigmoid

* add python

* add python api

* fix typo

* add doc

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