[SPARK-27621][ML] Linear Regression - validate training related params such as loss only during fitting phase

## What changes were proposed in this pull request?

When transform(...) method is called on a LinearRegressionModel created directly with the coefficients and intercepts, the following exception is encountered.

```
java.util.NoSuchElementException: Failed to find a default value for loss
	at org.apache.spark.ml.param.Params$$anonfun$getOrDefault$2.apply(params.scala:780)
	at org.apache.spark.ml.param.Params$$anonfun$getOrDefault$2.apply(params.scala:780)
	at scala.Option.getOrElse(Option.scala:121)
	at org.apache.spark.ml.param.Params$class.getOrDefault(params.scala:779)
	at org.apache.spark.ml.PipelineStage.getOrDefault(Pipeline.scala:42)
	at org.apache.spark.ml.param.Params$class.$(params.scala:786)
	at org.apache.spark.ml.PipelineStage.$(Pipeline.scala:42)
	at org.apache.spark.ml.regression.LinearRegressionParams$class.validateAndTransformSchema(LinearRegression.scala:111)
	at org.apache.spark.ml.regression.LinearRegressionModel.validateAndTransformSchema(LinearRegression.scala:637)
	at org.apache.spark.ml.PredictionModel.transformSchema(Predictor.scala:192)
	at org.apache.spark.ml.PipelineModel$$anonfun$transformSchema$5.apply(Pipeline.scala:311)
	at org.apache.spark.ml.PipelineModel$$anonfun$transformSchema$5.apply(Pipeline.scala:311)
	at scala.collection.IndexedSeqOptimized$class.foldl(IndexedSeqOptimized.scala:57)
	at scala.collection.IndexedSeqOptimized$class.foldLeft(IndexedSeqOptimized.scala:66)
	at scala.collection.mutable.ArrayOps$ofRef.foldLeft(ArrayOps.scala:186)
	at org.apache.spark.ml.PipelineModel.transformSchema(Pipeline.scala:311)
	at org.apache.spark.ml.PipelineStage.transformSchema(Pipeline.scala:74)
	at org.apache.spark.ml.PipelineModel.transform(Pipeline.scala:305)
```

This is because validateAndTransformSchema() is called both during training and scoring phases, but the checks against the training related params like loss should really be performed during training phase only, I think, please correct me if I'm missing anything :)

This issue was first reported for mleap (combust/mleap#455) because basically when we serialize the Spark transformers for mleap, we only serialize the params that are relevant for scoring. We do have the option to de-serialize the serialized transformers back into Spark for scoring again, but in that case, we no longer have all the training params.

## How was this patch tested?
Added a unit test to check this scenario.

Please let me know if there's anything additional required, this is the first PR that I've raised in this project.

Closes #24509 from ancasarb/linear_regression_params_fix.

Authored-by: asarb <asarb@expedia.com>
Signed-off-by: Sean Owen <sean.owen@databricks.com>
(cherry picked from commit 4241a72)
Signed-off-by: Sean Owen <sean.owen@databricks.com>

Author: asarb

mllib/src/main/scala/org/apache/spark/ml/regression/LinearRegression.scala

@@ -107,12 +107,13 @@ private[regression] trait LinearRegressionParams extends PredictorParams
       schema: StructType,
       fitting: Boolean,
       featuresDataType: DataType): StructType = {
-    if ($(loss) == Huber) {
-      require($(solver)!= Normal, "LinearRegression with huber loss doesn't support " +
-        "normal solver, please change solver to auto or l-bfgs.")
-      require($(elasticNetParam) == 0.0, "LinearRegression with huber loss only supports " +
-        s"L2 regularization, but got elasticNetParam = $getElasticNetParam.")
-
+    if (fitting) {
+      if ($(loss) == Huber) {
+        require($(solver)!= Normal, "LinearRegression with huber loss doesn't support " +
+          "normal solver, please change solver to auto or l-bfgs.")
+        require($(elasticNetParam) == 0.0, "LinearRegression with huber loss only supports " +
+          s"L2 regularization, but got elasticNetParam = $getElasticNetParam.")
+      }
     }
     super.validateAndTransformSchema(schema, fitting, featuresDataType)
   }

mllib/src/test/scala/org/apache/spark/ml/regression/LinearRegressionSuite.scala

@@ -182,6 +182,18 @@ class LinearRegressionSuite extends MLTest with DefaultReadWriteTest {
     assert(model.numFeatures === numFeatures)
   }
 
+  test("linear regression: can transform data with LinearRegressionModel") {
+    withClue("training related params like loss are only validated during fitting phase") {
+      val original = new LinearRegression().fit(datasetWithDenseFeature)
+
+      val deserialized = new LinearRegressionModel(uid = original.uid,
+        coefficients = original.coefficients,
+        intercept = original.intercept)
+      val output = deserialized.transform(datasetWithDenseFeature)
+      assert(output.collect().size > 0) // simple assertion to ensure no exception thrown
+    }
+  }
+
   test("linear regression: illegal params") {
     withClue("LinearRegression with huber loss only supports L2 regularization") {
       intercept[IllegalArgumentException] {