[SPARK-19400][ML] Allow GLM to handle intercept only model

## What changes were proposed in this pull request?
Intercept-only GLM is failing for non-Gaussian family because of reducing an empty array in IWLS. The following code `val maxTolOfCoefficients = oldCoefficients.toArray.reduce { (x, y) => math.max(math.abs(x), math.abs(y))` fails in the intercept-only model because `oldCoefficients` is empty. This PR fixes this issue.

yanboliang srowen imatiach-msft zhengruifeng

## How was this patch tested?
New test for intercept only model.

Author: actuaryzhang <actuaryzhang10@gmail.com>

Closes #16740 from actuaryzhang/interceptOnly.

mllib/src/main/scala/org/apache/spark/ml/optim/IterativelyReweightedLeastSquares.scala

@@ -89,7 +89,7 @@ private[ml] class IterativelyReweightedLeastSquares(
       val oldCoefficients = oldModel.coefficients
       val coefficients = model.coefficients
       BLAS.axpy(-1.0, coefficients, oldCoefficients)
-      val maxTolOfCoefficients = oldCoefficients.toArray.reduce { (x, y) =>
+      val maxTolOfCoefficients = oldCoefficients.toArray.foldLeft(0.0) { (x, y) =>
         math.max(math.abs(x), math.abs(y))
       }
       val maxTol = math.max(maxTolOfCoefficients, math.abs(oldModel.intercept - model.intercept))

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

@@ -335,6 +335,10 @@ class GeneralizedLinearRegression @Since("2.0.0") (@Since("2.0.0") override val
       throw new SparkException(msg)
     }
 
+    require(numFeatures > 0 || $(fitIntercept),
+      "GeneralizedLinearRegression was given data with 0 features, and with Param fitIntercept " +
+        "set to false. To fit a model with 0 features, fitIntercept must be set to true." )
+
     val w = if (!isDefined(weightCol) || $(weightCol).isEmpty) lit(1.0) else col($(weightCol))
     val instances: RDD[Instance] =
       dataset.select(col($(labelCol)), w, col($(featuresCol))).rdd.map {

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

@@ -743,6 +743,61 @@ class GeneralizedLinearRegressionSuite
     }
   }
 
+  test("generalized linear regression: intercept only") {
+    /*
+      R code:
+
+      library(statmod)
+      y <- c(1.0, 0.5, 0.7, 0.3)
+      w <- c(1, 2, 3, 4)
+      for (fam in list(gaussian(), poisson(), binomial(), Gamma(), tweedie(1.6))) {
+        model1 <- glm(y ~ 1, family = fam)
+        model2 <- glm(y ~ 1, family = fam, weights = w)
+        print(as.vector(c(coef(model1), coef(model2))))
+      }
+      [1] 0.625 0.530
+      [1] -0.4700036 -0.6348783
+      [1] 0.5108256 0.1201443
+      [1] 1.600000 1.886792
+      [1] 1.325782 1.463641
+     */
+
+    val dataset = Seq(
+      Instance(1.0, 1.0, Vectors.zeros(0)),
+      Instance(0.5, 2.0, Vectors.zeros(0)),
+      Instance(0.7, 3.0, Vectors.zeros(0)),
+      Instance(0.3, 4.0, Vectors.zeros(0))
+    ).toDF()
+
+    val expected = Seq(0.625, 0.530, -0.4700036, -0.6348783, 0.5108256, 0.1201443,
+      1.600000, 1.886792, 1.325782, 1.463641)
+
+    import GeneralizedLinearRegression._
+
+    var idx = 0
+    for (family <- Seq("gaussian", "poisson", "binomial", "gamma", "tweedie")) {
+      for (useWeight <- Seq(false, true)) {
+        val trainer = new GeneralizedLinearRegression().setFamily(family)
+        if (useWeight) trainer.setWeightCol("weight")
+        if (family == "tweedie") trainer.setVariancePower(1.6)
+        val model = trainer.fit(dataset)
+        val actual = model.intercept
+        assert(actual ~== expected(idx) absTol 1E-3, "Model mismatch: intercept only GLM with " +
+          s"useWeight = $useWeight and family = $family.")
+        assert(model.coefficients === new DenseVector(Array.empty[Double]))
+        idx += 1
+      }
+    }
+
+    // throw exception for empty model
+    val trainer = new GeneralizedLinearRegression().setFitIntercept(false)
+    withClue("Specified model is empty with neither intercept nor feature") {
+      intercept[IllegalArgumentException] {
+        trainer.fit(dataset)
+      }
+    }
+  }
+
   test("glm summary: gaussian family with weight") {
     /*
        R code: