Adds support for saving and loading nested ML Pipelines from Python. Pipeline and PipelineModel do not extend JavaWrapper, but they are able to utilize the JavaMLWriter, JavaMLReader implementations.
Also:
* Separates out interfaces from Java wrapper implementations for MLWritable, MLReadable, MLWriter, MLReader.
* Moves methods _stages_java2py, _stages_py2java into Pipeline, PipelineModel as _transfer_stage_from_java, _transfer_stage_to_java
Added new unit test for nested Pipelines. Abstracted validity check into a helper method for the 2 unit tests.
Author: Joseph K. Bradley <joseph@databricks.com>
Closes#11866 from jkbradley/nested-pipeline-io.
Closes#11835
7e3423b9c0