support NHWC format in SpatialAveragePooling (#1488)
support NHWC format in SpatialAveragePooling (#1488)

* support NHWC format in SpatialAveragePooling

* fix style

* meet code review

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