title: pandas transform one row into multiple rows
body: <p>I have a dataframe as below.</p>
  
  <p>My dataframe as below.</p>
  
  <pre><code>ID      list
  1       a, b, c
  2       a, s
  3       NA
  5       f, j, l
  </code></pre>
  
  <p>I need to break each items in the list column(String) into independent row as below: </p>
  
  <pre><code>ID      item
  1       a
  1       b
  1       c
  2       a
  2       s
  3       NA
  5       f
  5       j
  5       l
  </code></pre>
  
  <p>Thanks.</p>