private fun calculateGradients( inputs : DoubleArray , predY : Double , targetY : Double ) : Array<Any> {
    val dJ_dPred = meanSquaredErrorDerivative( predY , targetY )
    val dPred_dW = inputs
    val dJ_dW = MathOps.multiplyScalar( dPred_dW , dJ_dPred )
    val dJ_dB = dJ_dPred
    return arrayOf( dJ_dW , dJ_dB )
}

private fun meanSquaredErrorDerivative( predY : Double , targetY : Double ) : Double {
    return 2 * ( predY - targetY )
}