Prevent duplicate active stages (Production, Staging) on multiple imports into same registered model

[amesar]

If you import versions into a model with multiple import-model calls, MLflow UI semantics of having just one active stage were not being honored. Duplicate active stages (Production, Staging) were observed, i.e. two or more Production stages which cannot occur in the UI.

Problem: In import_model.py, the archive_existing_versions argument in the MlflowClient.transition_model_version_stage() call, was not set (default is False).

Fix: Set the archive_existing_versions argument to True to avoid multiple active stages.

See:

• MlflowClient.transition_model_version_stage() documentation page.
• https://github.com/mlflow/mlflow/issues/6414