[keras][loss] kullback_leibler_divergence and poisson (#1938)
[keras][loss] kullback_leibler_divergence and poisson (#1938)

* add kdl and poisson

* add extra unittest

* poisson update

* kullback

* update kld

* kld update

* style fix

* add transient

* add extra ut

* init with null

* resolve conflict

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