title: Difference between tf.layers.dense and tf.nn.xw_plus_b
body: <p>What is the difference between <code>tf.layers.dense</code> and <code>tf.nn.xw_plus_b</code> in <code>TF</code>? 
  What is the default activation used in <code>tf.layers.dense</code> when "activation" argument is passed as <code>None</code>?                                </p>