Optimize padding mechanism (#2324)
Optimize padding mechanism (#2324)

* optimize padding options

* restore classNLLCriterion comments

* meet code review

* add more tests

* meet code review

* meet code review

* meet code review

* Add python api

* fix a bug

* fix a bug

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