Add horovod estimator save/load/get_models/shutdown (#2702)
Add horovod estimator save/load/get_models/shutdown (#2702)

* add get_model/save/load/shutdown for pytorch horovod estimator

* change args

* remove condition

* change name

* remove get multiple models

* add pytorch estimator ut and disable for now

* meet comments
intel-analytics/BigDL,intel-analytics/BigDL,yangw1234/BigDL,yangw1234/BigDL,yangw1234/BigDL,yangw1234/BigDL,intel-analytics/BigDL,intel-analytics/BigDL