title: Pandas groupby split by NaN
body: <p>I have one question, I ask you guys</p>
  <p>I want to split the df like grp1, grp2, grp3, grp4</p>
  <p>The data below is a sample and I really need to process a large amount of data, so please let me know how to do it appropriately.</p>
  <pre><code>df = 
                              A1     B1
      2021-03-18 00:00:00     10     23
      2021-03-18 00:00:01     11     28 
      2021-03-18 00:00:02     12     29
      2021-03-18 00:00:03     NaN    NaN
      2021-03-18 00:00:04     NaN    NaN
      2021-03-18 00:00:05     42     32
      2021-03-18 00:00:06     51     90 
      2021-03-18 00:00:07     NaN    NaN
      2021-03-18 00:00:08     90     101
      2021-03-18 00:00:09     32     42
      2021-03-18 00:00:10     NaN    NaN
      2021-03-18 00:00:11     575    333
      2021-03-18 00:00:12     62     421
  </code></pre>
  <pre><code>grp1 =
                                A1     B1
        2021-03-18 00:00:00     10     23
        2021-03-18 00:00:01     11     28 
        2021-03-18 00:00:02     12     29
  </code></pre>
  <pre><code>grp2 =
                                A1     B1
        2021-03-18 00:00:05     42     32
        2021-03-18 00:00:06     51     90 
  </code></pre>
  <pre><code>grp3 =
                                A1     B1
        2021-03-18 00:00:08     90     101
        2021-03-18 00:00:09     32     42
  </code></pre>
  <pre><code>grp4 =
                                A1     B1
        2021-03-18 00:00:11     575    333
        2021-03-18 00:00:12     62     421
  </code></pre>