[jvm-packages] support spark 2.4 and compatibility test with previous xgboost version (#4377)

* bump spark version

* keep float.nan

* handle brokenly changed name/value

* add test

* add model files

* add model files

* update doc

jvm-packages/xgboost4j-spark/src/main/scala/ml/dmlc/xgboost4j/scala/spark/params/DefaultXGBoostParamsReader.scala

@@ -22,12 +22,17 @@ import org.json4s.JsonAST.JObject
 import org.json4s.jackson.JsonMethods.{compact, parse, render}
 
 import org.apache.spark.SparkContext
-import org.apache.spark.ml.param.Params
+import org.apache.spark.ml.param.{Param, Params}
 import org.apache.spark.ml.util.MLReader
 
 // This originates from apache-spark DefaultPramsReader copy paste
 private[spark] object DefaultXGBoostParamsReader {
 
+  private val paramNameCompatibilityMap: Map[String, String] = Map("silent" -> "verbosity")
+
+  private val paramValueCompatibilityMap: Map[String, Map[Any, Any]] =
+    Map("objective" -> Map("reg:linear" -> "reg:squarederror"))
+
   /**
    * All info from metadata file.
    *
@@ -103,6 +108,14 @@ private[spark] object DefaultXGBoostParamsReader {
     Metadata(className, uid, timestamp, sparkVersion, params, metadata, metadataStr)
   }
 
+  private def handleBrokenlyChangedValue[T](paramName: String, value: T): T = {
+    paramValueCompatibilityMap.getOrElse(paramName, Map()).getOrElse(value, value).asInstanceOf[T]
+  }
+
+  private def handleBrokenlyChangedName(paramName: String): String = {
+    paramNameCompatibilityMap.getOrElse(paramName, paramName)
+  }
+
   /**
    * Extract Params from metadata, and set them in the instance.
    * This works if all Params implement [[org.apache.spark.ml.param.Param.jsonDecode()]].
@@ -113,9 +126,9 @@ private[spark] object DefaultXGBoostParamsReader {
     metadata.params match {
       case JObject(pairs) =>
         pairs.foreach { case (paramName, jsonValue) =>
-          val param = instance.getParam(paramName)
+          val param = instance.getParam(handleBrokenlyChangedName(paramName))
           val value = param.jsonDecode(compact(render(jsonValue)))
-          instance.set(param, value)
+          instance.set(param, handleBrokenlyChangedValue(paramName, value))
         }
       case _ =>
         throw new IllegalArgumentException(