refine tensorflow support doc (#1253)
refine tensorflow support doc (#1253)

* refine tensorflow support

* meet review

* add tensorflow saver to python api and refine docs

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