[SPARK-8994] [ML] tiny cleanups to Params, Pipeline

Made default impl of Params.validateParams empty
CC mengxr

Author: Joseph K. Bradley <joseph@databricks.com>

Closes #7349 from jkbradley/pipeline-small-cleanups and squashes the following commits:

4e0f013 [Joseph K. Bradley] small cleanups after SPARK-5956

mllib/src/main/scala/org/apache/spark/ml/Pipeline.scala

@@ -95,6 +95,8 @@ class Pipeline(override val uid: String) extends Estimator[PipelineModel] {
   /** @group setParam */
   def setStages(value: Array[PipelineStage]): this.type = { set(stages, value); this }
 
+  // Below, we clone stages so that modifications to the list of stages will not change
+  // the Param value in the Pipeline.
   /** @group getParam */
   def getStages: Array[PipelineStage] = $(stages).clone()
 

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

@@ -341,9 +341,7 @@ trait Params extends Identifiable with Serializable {
    * those are checked during schema validation.
    */
   def validateParams(): Unit = {
-    params.filter(isDefined).foreach { param =>
-      param.asInstanceOf[Param[Any]].validate($(param))
-    }
+    // Do nothing by default.  Override to handle Param interactions.
   }
 
   /**