title: What is the relation between validation_data and validation_split in Keras' fit function?
body: <p><code>validation_split</code> says: "<em>hey give me all the input data – I will take care of splitting between test and validation</em>".</p>
  
  <pre><code>model.fit(inputX, inputY, validation_split=0.20, epochs=10, batch_size=10)
  </code></pre>
  
  <p><code>validation_data</code> says "<em>please give me explicitly the validation data</em>"</p>
  
  <pre><code>model.fit(inputX, inputY, validation_data=(testX,testY), epochs=10, batch_size=10)
  </code></pre>
  
  <p>Is there any hidden trick or something I am missing apart from my understanding?</p>