Graduate MPI Operator, MXNet Operator and XGBoost Operator to v1

[terrytangyuan]

This is part of the roadmap.

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[terrytangyuan]

Progress are being tracked in individual repos:

• MXNet Operator Graduate mxnet-operator to v1 mxnet-operator#66
• MPI Operator Graduate MPI Operator to v1 mpi-operator#217
• XGBoost Operator Graduate XGBoost Operator to v1 xgboost-operator#44

[richardsliu]

As a reference, these are the graduation criteria for TFJob 1.0: kubeflow/training-operator#1076

[terrytangyuan]

Also here are some good references on criteria, processes, and past exits: https://github.com/kubeflow/community/blob/master/guidelines/application_requirements.md#reference