Update UTs and examples with init_orca_context (#2787)
Update UTs and examples with init_orca_context (#2787)

* update unit tests

* minor

* update

* update mxnet

* move barrier

* fix mxnet

* update

* bug fix

* update

* update test

* update mxnet example

* update mxnet

* minor

* minor

* minor

* update examples

* move ray import dependencies

* readme

* minor

* bug fix

* remove default
yangw1234/BigDL,yangw1234/BigDL,intel-analytics/BigDL,intel-analytics/BigDL,yangw1234/BigDL,yangw1234/BigDL,intel-analytics/BigDL,intel-analytics/BigDL