Change MXNetTrainer to Estimator and remove RDD from plasma after training finishes (#2495)
Change MXNetTrainer to Estimator and remove RDD from plasma after training finishes (#2495)

* initial

* change to estimator

* style

* change to fit

* meet review

* refine ut

* update

* handle error

* meet review

* style

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