title: How to Use AutoGrad Packages?
body: <p>I am trying to do a simple thing: use autograd to get gradients and do gradient descent:</p>
  
  <pre><code>import tangent
  
  def model(x):
      return a*x + b
  
  def loss(x,y):
      return (y-model(x))**2.0
  </code></pre>
  
  <p>After getting loss for an input-output pair, I want to get gradients wrt loss:</p>
  
  <pre><code>    l = loss(1,2)
      # grad_a = gradient of loss wrt a?
      a = a - grad_a
      b = b - grad_b
  </code></pre>
  
  <p>But the library tutorials don't show how to do obtain gradient with respect to a or b i.e. the parameters so, neither autograd nor tangent.</p>