Add document for wrap preprocessor and model in one graph and add its python API (#2595)
Add document for wrap preprocessor and model in one graph and add its python API (#2595)

* add document for wrap preprocessor and model in one graph and add its python API

* enable junit report for pytest

* fix failed unit test

* fix unit test

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