title: What does -1 in numpy reshape mean?
body: <pre><code> I have a numpy array (A) of shape = (100000, 28, 28)
   I reshape it using A.reshape(-1, 28x28)
  </code></pre>
  
  <p>This is very common use in Machine learning pipelines.
  How does this work ? I have never understood the meaning of '-1' in reshape. </p>
  
  <p>An exact question is <a href="https://stackoverflow.com/questions/18691084/what-does-1-mean-in-numpy-reshape">this</a>
  But no solid explanation. Any answers pls ? </p>