## What changes were proposed in this pull request?
We save ML's user-supplied params and default params as one entity in metadata. During loading the saved models, we set all the loaded params into created ML model instances as user-supplied params.
It causes some problems, e.g., if we strictly disallow some params to be set at the same time, a default param can fail the param check because it is treated as user-supplied param after loading.
The loaded default params should not be set as user-supplied params. We should save ML default params separately in metadata.
For backward compatibility, when loading metadata, if it is a metadata file from previous Spark, we shouldn't raise error if we can't find the default param field.
## How was this patch tested?
Pass existing tests and added tests.
Author: Liang-Chi Hsieh <viirya@gmail.com>
Closes#20633 from viirya/save-ml-default-params.
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