title: Pytorch lightning metrics: ValueError: preds and target must have same number of dimensions, or one additional dimension for preds
body: <p>Googling this gets you no where, so I decided to help future me and others by posting this as a searchable question.</p>
  <hr />
  <pre><code>def __init__():
      ...
      self.val_acc = pl.metrics.Accuracy()
  
  def validation_step(self, batch, batch_index):
      ...
      self.val_acc.update(log_probs, label_batch)
  </code></pre>
  <p>gives</p>
  <pre><code>ValueError: preds and target must have same number of dimensions, or one additional dimension for preds
  </code></pre>
  <p>for <code>log_probs.shape == (16, 4)</code> and for <code>label_batch.shape == (16, 4)</code></p>
  <p>What's the issue?</p>