title: How to understand this: `db = np.sum(dscores, axis=0, keepdims=True)`
body: <p>In <a href="http://cs231n.github.io/neural-networks-case-study/" rel="nofollow noreferrer">cs231n 2017 class</a>, when we backpropagate the gradient we update the biases like this:</p>
  
  <pre><code>db = np.sum(dscores, axis=0, keepdims=True)
  </code></pre>
  
  <p>What's the basic idea behind the <code>sum</code> operation? Thanks</p>