title: reduce from PySpark RDD returns tuple
body: <pre><code>data = data.withColumn('n', F.lit(10))
  result = data.select('n').rdd.reduce(lambda x, y: x + y)
  print(result)
  </code></pre>
  
  <p>Above code returns following output. But I was expecting single value which is the sum.</p>
  
  <pre><code>output:
  (10,
   10,
   10,
   10,
   10,
   10,
   10,
   10,.....)
  
  </code></pre>