## What changes were proposed in this pull request?

Constraint propagation can be computation expensive and block the driver execution for long time. For example, the below benchmark needs 30mins.

Compared with previous PRs #16998, #16785, this is a much simpler option: add a flag to disable constraint propagation.

### Benchmark

Run the following codes locally.

    import org.apache.spark.ml.{Pipeline, PipelineStage}
    import org.apache.spark.ml.feature.{OneHotEncoder, StringIndexer, VectorAssembler}
    import org.apache.spark.sql.internal.SQLConf

    spark.conf.set(SQLConf.CONSTRAINT_PROPAGATION_ENABLED.key, false)

    val df = (1 to 40).foldLeft(Seq((1, "foo"), (2, "bar"), (3, "baz")).toDF("id", "x0"))((df, i) => df.withColumn(s"x$i", $"x0"))

    val indexers = df.columns.tail.map(c => new StringIndexer()
      .setInputCol(c)
      .setOutputCol(s"${c}_indexed")
      .setHandleInvalid("skip"))

    val encoders = indexers.map(indexer => new OneHotEncoder()
      .setInputCol(indexer.getOutputCol)
      .setOutputCol(s"${indexer.getOutputCol}_encoded")
      .setDropLast(true))

    val stages: Array[PipelineStage] = indexers ++ encoders
    val pipeline = new Pipeline().setStages(stages)

    val startTime = System.nanoTime
    pipeline.fit(df).transform(df).show
    val runningTime = System.nanoTime - startTime

Before this patch: 1786001 ms ~= 30 mins
After this patch: 26392 ms = less than half of a minute

Related PRs: #16998, #16785.

## How was this patch tested?

Jenkins tests.

Please review http://spark.apache.org/contributing.html before opening a pull request.

Author: Liang-Chi Hsieh <viirya@gmail.com>

Closes #17186 from viirya/add-flag-disable-constraint-propagation.