Add memory type support for Orca tf estimator (#3280)
Add memory type support for Orca tf estimator (#3280)

* add mem type for dataframe dataset

* add ZooTrigger support

* update mem type

* add test

* update

* update mem type and orca context

* update orca context docs

* update orca context with comments

* fix style
yangw1234/BigDL,yangw1234/BigDL,yangw1234/BigDL,intel-analytics/BigDL,intel-analytics/BigDL,intel-analytics/BigDL,yangw1234/BigDL,intel-analytics/BigDL