title: How to calculate vector-wise dot product in Keras?
body: <p>I hope to calculate a vector wise dot product in Keras. 
  In detail, I mean if I have two tensor <code>A</code> and <code>B</code>, both with shape <code>(None, 30, 100)</code>, I want to calculate the result C with shape <code>(None, 30, 1)</code> which would satisfy </p>
  
  <pre><code>C[:,:,i] = dot(A[:,:,i], B[:,:,i]).
  </code></pre>
  
  <p>I wonder if that is possible, since the <code>batch_dot()</code> function in the backend would only return the shape <code>(None, 30, 30)</code> and only have the relationship </p>
  
  <pre><code>C[:,i,j] = dot(A[:,:,i], B[:,:,j])
  </code></pre>
  
  <p>But that is not what I want.</p>
  
  <p>Thank you!</p>