[SPARK-29381][PYTHON][ML] Add _ before the XXXParams classes

[huaxingao]

What changes were proposed in this pull request?

Add _ before XXXParams classes to indicate internal usage

Why are the changes needed?

Follow the PEP 8 convention to use _single_leading_underscore to indicate internal use

Does this PR introduce any user-facing change?

No

How was this patch tested?

use existing tests

[SparkQA]

Test build #111976 has finished for PR 26103 at commit 3a5962c.

• This patch passes all tests.
• This patch merges cleanly.
• This patch adds the following public classes (experimental):
• class _JavaClassifierParams(HasRawPredictionCol, _JavaPredictorParams):
• class JavaClassifier(JavaPredictor, _JavaClassifierParams):
• class JavaClassificationModel(JavaPredictionModel, _JavaClassifierParams):
• class _JavaProbabilisticClassifierParams(HasProbabilityCol, HasThresholds, _JavaClassifierParams):
• class JavaProbabilisticClassifier(JavaClassifier, _JavaProbabilisticClassifierParams):
• class _GBTClassifierParams(_GBTParams, _HasVarianceImpurity):
• class GBTClassifier(JavaProbabilisticClassifier, _GBTClassifierParams,
• class _OneVsRestParams(_JavaClassifierParams, HasWeightCol):
• class OneVsRest(Estimator, _OneVsRestParams, HasParallelism, JavaMLReadable, JavaMLWritable):
• class OneVsRestModel(Model, _OneVsRestParams, JavaMLReadable, JavaMLWritable):
• class _JavaPredictorParams(HasLabelCol, HasFeaturesCol, HasPredictionCol):
• class JavaPredictor(JavaEstimator, _JavaPredictorParams):
• class JavaPredictionModel(JavaModel, _JavaPredictorParams):

[srowen]

Would this affect anyone who has subclassed these quasi-internal classes? I'm trying to figure out how likely it is that this breaks someone's extension vs the value. We can break this in 3.0 but just weigh it against how important it is.

[huaxingao]

Almost all of these quasi-internal classes _XXXParams are newly added in these parity jiras with very few exceptions. One of them is LSHParams. If user has subclass this LSHParams, with the name changed to _LSHParams, user has to explicitly import this class like from pyspark.ml.feature import _LSHParamsbecause import * does not import objects whose names start with an underscore.

[srowen]

Seems OK pending tests.

[SparkQA]

Test build #4896 has finished for PR 26103 at commit 3a5962c.

• This patch passes all tests.
• This patch merges cleanly.
• This patch adds the following public classes (experimental):
• class _JavaClassifierParams(HasRawPredictionCol, _JavaPredictorParams):
• class JavaClassifier(JavaPredictor, _JavaClassifierParams):
• class JavaClassificationModel(JavaPredictionModel, _JavaClassifierParams):
• class _JavaProbabilisticClassifierParams(HasProbabilityCol, HasThresholds, _JavaClassifierParams):
• class JavaProbabilisticClassifier(JavaClassifier, _JavaProbabilisticClassifierParams):
• class _GBTClassifierParams(_GBTParams, _HasVarianceImpurity):
• class GBTClassifier(JavaProbabilisticClassifier, _GBTClassifierParams,
• class _OneVsRestParams(_JavaClassifierParams, HasWeightCol):
• class OneVsRest(Estimator, _OneVsRestParams, HasParallelism, JavaMLReadable, JavaMLWritable):
• class OneVsRestModel(Model, _OneVsRestParams, JavaMLReadable, JavaMLWritable):
• class _JavaPredictorParams(HasLabelCol, HasFeaturesCol, HasPredictionCol):
• class JavaPredictor(JavaEstimator, _JavaPredictorParams):
• class JavaPredictionModel(JavaModel, _JavaPredictorParams):

[srowen]

Merged to master

[huaxingao]

Thanks a lot! @srowen