title: What does fit, transform, and fit_transform do in PCA available in sklearn.decomposition?
body: <p>I am trying to mimic the behavior of <code>PCA</code> class available in <code>sklearn.decomposition</code>. </p>
  
  <p>I have wrote a method which computes the SVD but I am not sure what does <code>fit()</code>, <code>tranform()</code>, and <code>fit_transform()</code> do without which I'm not able to proceed further. </p>
  
  <p>I think <code>fit()</code> computes the svd and the singular values can be accessed using the <code>singular_values_</code> attribute but I don't know about the remaining two methods.</p>