[New feature] add transformer layer (#2825)
[New feature] add transformer layer (#2825)

* add transformer

* refactor class name

* use same embedding for translation

* fix pr comments

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