title: Spark AccumulatorV2 with HashMap
body: <p>I am trying to create a custom AccumulatorV2 with a hash map, the input would be hashmap and output would be a map of HashMap, </p>
  
  <p>My intention is to have a K -> K1,V, where the value will increment. I am confused by the scala syntax for overriding AccumulatorV2 for Map, did anyone had a luck with this.</p>
  
  <pre><code>class CustomAccumulator extends AccumulatorV2[java.util.Map[String, String], java.util.Map[String,java.util.Map[String, Double]]]
  </code></pre>