Add Cropping2D and Cropping3D (#1992)
Add Cropping2D and Cropping3D (#1992)

* add cropping

* serialization test

* equal and hash

* style fix

* python doc

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