title: Kafka does not create consumer group when reading from single partition
body: <p>I am reading from kafka using spark structured streaming, below is my read stream</p>
  
  <pre><code> val inputDf = spark.readStream
    .format("kafka")
    .option("kafka.bootstrap.servers", brokers)
   //.option("subscribe", topic)
   .option("assign"," {\""+topic+"\":[0]}") 
    .option("startingOffsets", "earliest")
    .option("maxOffsetsPerTrigger", 60000)
    .load()
  </code></pre>
  
  <p>I'm trying to see my consumer group in kafka using below kafka command. What I've noticed is when i'm reading from only single partition it does not create any comsumer group but when i read from topic(in my case I have 3 partitions for the topic) without mentioning partitions explicitly it create consumer groups. Is this an expected behavior?If yes, then why?</p>
  
  <pre><code>./kafka-consumer-groups.sh --bootstrap-server kfk01.sboxdc.com:9092,kfk02.sboxdc.com:9092,kfk03.sboxdc.com:9092 --list
  </code></pre>