add --data_path option (#3981)
add --data_path option (#3981)

* add --data_path option

* set default value '/tmp/mnist' for data_path

Co-authored-by: shaojie <cd9162835afa154941a3d7d99d801a528eb1bd22@intel.com>
intel-analytics/BigDL,intel-analytics/BigDL,yangw1234/BigDL,yangw1234/BigDL,intel-analytics/BigDL,yangw1234/BigDL,intel-analytics/BigDL,yangw1234/BigDL