title: sklearn StandardScaler, doesn't allow direct transform, we need to fit_transform
body: <p>What is the difference between fit_transform and transform?
  Why doesn't transform directly works?</p>
  
  <pre><code>from sklearn.preprocessing import StandardScaler
  
   X_scaler = StandardScaler()
   X_train = X_scaler.fit_transform(X_train)
   X_test = X_scaler.transform(X_test)
  </code></pre>
  
  <p>If directly transformed it gives the below error</p>
  
  <blockquote>
    <p>NotFittedError: This StandardScaler instance is not fitted yet. Call
    'fit' with appropriate arguments before using this method.</p>
  </blockquote>