[New feature] Add attention layer and ffn layer (#2795)
[New feature] Add attention layer and ffn layer (#2795)

* add attention layer

* add ffn layer and more unit tests

* refactor according to pr comments

* add SerializationTest

* fix unit tests

* add python api

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