transfer learning api (#160)
transfer learning api (#160)

* transfer learning

* add tests

* refine example

* add python example

* change package

* address comments

* fix style

* fix toModel

* fix tests

* fix style

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