Support global average pooling (#1365)
Support global average pooling (#1365)

* add global pooling

* update doc and python

* update doc

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