Missing persist on sources in mllib.feature.PCA

  def fit(sources: RDD[Vector]): PCAModel = {
    // first use rdd sources on action first()
    val numFeatures = sources.first().size
    require(k <= numFeatures,
      s"source vector size $numFeatures must be no less than k=$k")
    require(PCAUtil.memoryCost(k, numFeatures) < Int.MaxValue,
      "The param k and numFeatures is too large for SVD computation. " +
      "Try reducing the parameter k for PCA, or reduce the input feature " +
      "vector dimension to make this tractable.")

    val mat = new RowMatrix(sources)
    // second use rdd sources
    val (pc, explainedVariance) = mat.computePrincipalComponentsAndExplainedVariance(k)