title: Where is the tensorflow session in Keras
body: <p>I'm new to keras and tensorflow. 
  When I write programs with tensorflow, I must bulid a session to run the graph. However, when I use keras, although the backend is obviously tensorflow, I don't see session in the keras code. It seems all thing is done after the model.compile and model.fit.</p>
  
  <p>So, how does Keras work? where is the tensorflow session? and instead of session, can I use eager execution with keras?</p>
  
  <p>Thanks in advance and sorry for my English</p>