[SPARK-9841] [ML] Make clear public

It is currently impossible to clear Param values once set. It would be helpful to be able to.

Author: Holden Karau <holden@pigscanfly.ca>

Closes #8619 from holdenk/SPARK-9841-params-clear-needs-to-be-public.

mllib/src/main/scala/org/apache/spark/ml/param/params.scala

@@ -454,7 +454,7 @@ trait Params extends Identifiable with Serializable {
   /**
    * Clears the user-supplied value for the input param.
    */
-  protected final def clear(param: Param[_]): this.type = {
+  final def clear(param: Param[_]): this.type = {
     shouldOwn(param)
     paramMap.remove(param)
     this

mllib/src/test/scala/org/apache/spark/ml/param/ParamsSuite.scala

@@ -156,6 +156,11 @@ class ParamsSuite extends SparkFunSuite {
     solver.clearMaxIter()
     assert(!solver.isSet(maxIter))
 
+    // Re-set and clear maxIter using the generic clear API
+    solver.setMaxIter(10)
+    solver.clear(maxIter)
+    assert(!solver.isSet(maxIter))
+
     val copied = solver.copy(ParamMap(solver.maxIter -> 50))
     assert(copied.uid === solver.uid)
     assert(copied.getInputCol === solver.getInputCol)