title: Train only some of the variables in tensorflow
body: <p>I'm using tensorflow to do a gradient decent classification. </p>
  
  <pre><code>train_op = tf.train.GradientDescentOptimizer(learning_rate).minimize(cost)
  </code></pre>
  
  <p>here <code>cost</code> is the <strong>cost function</strong> that I have used in optimization. 
  After launching the Graph in the Session, the Graph can be fed as:</p>
  
  <pre><code>sess.run(train_op, feed_dict)
  </code></pre>
  
  <p>And with this, all the variables in the cost function will be updated in order to minimized the cost.</p>
  
  <p>Here is my question. How can I update only some variables in the cost function when training..? Is there a way to convert created variables into constants or something..?</p>