title: how to determine which Merge mode (add/ average/ multiply/ dot / concat) to use?
body: <p>After testing the script of [babi_rnn.py] and [babi_memnn.py], the question of [how to determine which Merge mode (add/ average/ multiply/ dot / concat) to use?] raised up many times in my mind.</p>
  
  <p>For example, for the LSTM modeling，it seems easy to understand that using [concat] to merge let's say two-branches's time sequence layer output.</p>
  
  <p>However, it is not that easy for me to understand why to use [add] to merge two branches in [babi_rnn.py]. In [babi_memnn.py], the [add],[dot] and [concat] merging modes are recruited.</p>
  
  <p>So, is there any suggestions for choosing which merging function to use in different usage scenarios?</p>