Add TransformerCriterion (#2097)
Add TransformerCriterion (#2097)

* add TransformerCriterion

* fix style

* fix python tests

* meet review

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