A dataset can store a column as sequence of Vectors but not directly vectors

A dataset can store an array of vectors but not a vector. This is inconsistent.

To reproduce:

    import org.apache.spark.ml.linalg.SQLDataTypes.VectorType
    import org.apache.spark.sql.types._
    import spark.implicits._

    val rdd = sc.parallelize(Seq(Row(Seq(Vectors.dense(Array(1.0, 2.0)).toSparse))))
    val arrayOfVectorsDS = spark.createDataFrame(rowRDD= rdd, schema = new StructType(Array(StructField(name = "value", dataType = ArrayType(elementType = VectorType))))).as[Seq[Vector]]
//    val vectorsDS = arrayOfVectorsDS.flatMap(a => a)
    .show
}

 If the line before ".show" is uncommented this code will throw the well known error: error: Unable to find encoder for type stored in a Dataset. Primitive types (Int, String, etc) and Product types (case classes) are supported by importing spark.implicits._ Support for serializing other types will be added in future releases.