title: How does BERT word embedding preprocess work
body: <p>I'm trying to figure out what <code>BERT</code> preprocess does. I mean, how it is done. But I can't find a good explanation. I would appreciate, if somebody know, a link to a better and deeply explained solution.
  If someone, by the other hand, wants to solve it here, I would be also extremly thankful!</p>
  <p>My question is, how does <code>BERT</code> mathematically convert a string input into a vector of numbers with fixed size? Which are the logical steps that follows?</p>