title: Usage of tf.GraphKeys
body: <p>In tensorflow, there's a class <code>GraphKeys</code>. I came across many codes, where it's been used. But it's not explained very well what's the usage of this class both in tensorflow documentation as well as in the codes, where it has been used.</p>
  
  <p>Can someone please explain what's the usage of <code>tf.GraphKey</code>?</p>
  
  <p>Thank you!</p>