[SPARK-11842] [ML] Small cleanups to existing Readers and Writers

[jkbradley]

Updates:

• Add repartition(1) to save() methods' saving of data for LogisticRegressionModel, LinearRegressionModel.
• Strengthen privacy to class and companion object for Writers and Readers
• Change LogisticRegressionSuite read/write test to fit intercept
• Add Since versions for read/write methods in Pipeline, LogisticRegression
• Switch from hand-written class names in Readers to using getClass

CC: @mengxr

CC: @yanboliang Would you mind taking a look at this PR? @mengxr might not be able to soon. Thank you!

[jkbradley]

Also @yanboliang please ping me if you send other read/write PRs

[SparkQA]

Test build #46288 has finished for PR 9829 at commit f042948.

• This patch passes all tests.
• This patch merges cleanly.
• This patch adds the following public classes (experimental):\n * public final class UnsafeSorterSpillReader extends UnsafeSorterIterator implements Closeable\n * class Pipeline(override val uid: String) extends Estimator[PipelineModel] with MLWritable\n * class LogisticRegressionModelWriter(instance: LogisticRegressionModel)\n * class CountVectorizerModelWriter(instance: CountVectorizerModel) extends MLWriter\n * final class IDF(override val uid: String) extends Estimator[IDFModel] with IDFBase\n * class MinMaxScalerModelWriter(instance: MinMaxScalerModel) extends MLWriter\n * class SQLTransformer @Since(\"1.6.0\") (override val uid: String) extends Transformer\n * class StandardScalerModelWriter(instance: StandardScalerModel) extends MLWriter\n * class StringIndexModelWriter(instance: StringIndexerModel) extends MLWriter\n * class ALS(override val uid: String) extends Estimator[ALSModel] with ALSParams\n * abstract class MLWriter extends BaseReadWrite with Logging\n * trait MLWritable\n * abstract class MLReader[T] extends BaseReadWrite\n * trait MLReadable[T]\n * trait ScalaReflection\n * case class Schema(dataType: DataType, nullable: Boolean)\n * s\"Unable to generate an encoder for inner class$\n *case class EncodeUsingSerializer(child: Expression, kryo: Boolean) extends UnaryExpression \n *case class DecodeUsingSerializer[T](child: Expression, tag: ClassTag[T], kryo: Boolean)`\n

[SparkQA]

Test build #46291 has finished for PR 9829 at commit 590f62f.

• This patch passes all tests.
• This patch merges cleanly.
• This patch adds the following public classes (experimental):\n * class LogisticRegressionModelWriter(instance: LogisticRegressionModel)\n

[mengxr]

LGTM. Merged into master and branch-1.6. Thanks!