Fix MXNet example download issue (#2215)
Fix MXNet example download issue (#2215)

* change to 1 worker

* fix data download

* update
intel-analytics/BigDL,yangw1234/BigDL,intel-analytics/BigDL,intel-analytics/BigDL,yangw1234/BigDL,yangw1234/BigDL,intel-analytics/BigDL,yangw1234/BigDL