Support Training using String Data (#1833)
Support Training using String Data (#1833)

* support string input training

* disable map dataset

* fix tests

* disable MapDataset in Keras training

* fix test

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