[SPARK-3724][ML] RandomForest: More options for feature subset size.

## What changes were proposed in this pull request?

This PR tries to support more options for feature subset size in RandomForest implementation. Previously, RandomForest only support "auto", "all", "sort", "log2", "onethird". This PR tries to support any given value to allow model search.

In this PR, `featureSubsetStrategy` could be passed with:
a) a real number in the range of `(0.0-1.0]` that represents the fraction of the number of features in each subset,
b)  an integer number (`>0`) that represents the number of features in each subset.

## How was this patch tested?

Two tests `JavaRandomForestClassifierSuite` and `JavaRandomForestRegressorSuite` have been updated to check the additional options for params in this PR.
An additional test has been added to `org.apache.spark.mllib.tree.RandomForestSuite` to cover the cases in this PR.

Author: Yong Tang <yong.tang.github@outlook.com>

Closes #11989 from yongtang/SPARK-3724.

1 files changed, 36 insertions, 0 deletions

diff --git a/mllib/src/test/scala/org/apache/spark/ml/tree/impl/RandomForestSuite.scala b/mllib/src/test/scala/org/apache/spark/ml/tree/impl/RandomForestSuite.scala
index cd402b1e1f..6db9ce150d 100644
--- a/mllib/src/test/scala/org/apache/spark/ml/tree/impl/RandomForestSuite.scala
+++ b/mllib/src/test/scala/org/apache/spark/ml/tree/impl/RandomForestSuite.scala
@@ -426,12 +426,48 @@ class RandomForestSuite extends SparkFunSuite with MLlibTestSparkContext {
       (math.log(numFeatures) / math.log(2)).ceil.toInt)
     checkFeatureSubsetStrategy(numTrees = 1, "onethird", (numFeatures / 3.0).ceil.toInt)
 
+    val realStrategies = Array(".1", ".10", "0.10", "0.1", "0.9", "1.0")
+    for (strategy <- realStrategies) {
+      val expected = (strategy.toDouble * numFeatures).ceil.toInt
+      checkFeatureSubsetStrategy(numTrees = 1, strategy, expected)
+    }
+
+    val integerStrategies = Array("1", "10", "100", "1000", "10000")
+    for (strategy <- integerStrategies) {
+      val expected = if (strategy.toInt < numFeatures) strategy.toInt else numFeatures
+      checkFeatureSubsetStrategy(numTrees = 1, strategy, expected)
+    }
+
+    val invalidStrategies = Array("-.1", "-.10", "-0.10", ".0", "0.0", "1.1", "0")
+    for (invalidStrategy <- invalidStrategies) {
+      intercept[MatchError]{
+        val metadata =
+          DecisionTreeMetadata.buildMetadata(rdd, strategy, numTrees = 1, invalidStrategy)
+      }
+    }
+
     checkFeatureSubsetStrategy(numTrees = 2, "all", numFeatures)
     checkFeatureSubsetStrategy(numTrees = 2, "auto", math.sqrt(numFeatures).ceil.toInt)
     checkFeatureSubsetStrategy(numTrees = 2, "sqrt", math.sqrt(numFeatures).ceil.toInt)
     checkFeatureSubsetStrategy(numTrees = 2, "log2",
       (math.log(numFeatures) / math.log(2)).ceil.toInt)
     checkFeatureSubsetStrategy(numTrees = 2, "onethird", (numFeatures / 3.0).ceil.toInt)
+
+    for (strategy <- realStrategies) {
+      val expected = (strategy.toDouble * numFeatures).ceil.toInt
+      checkFeatureSubsetStrategy(numTrees = 2, strategy, expected)
+    }
+
+    for (strategy <- integerStrategies) {
+      val expected = if (strategy.toInt < numFeatures) strategy.toInt else numFeatures
+      checkFeatureSubsetStrategy(numTrees = 2, strategy, expected)
+    }
+    for (invalidStrategy <- invalidStrategies) {
+      intercept[MatchError]{
+        val metadata =
+          DecisionTreeMetadata.buildMetadata(rdd, strategy, numTrees = 2, invalidStrategy)
+      }
+    }
   }
 
   test("Binary classification with continuous features: subsampling features") {