title: validation_epoch_end with DDP Pytorch Lightning
body: <p>What is your question?</p>
  <p>I am trying to implement a metric which needs access to whole data. So instead of updating the metric in *_step() methods, I am trying to collect the outputs in the *_epoch_end() methods. However, the outputs contain only the output of the partition of the data each device gets. Basically if there are n devices, then each device is getting 1/n of the total outputs.</p>
  <p>What's your environment?</p>
  <pre><code>OS: ubuntu
  Packaging: conda
  Version [1.0.4
  Pytorch: 1.6.0
  </code></pre>