title: sklearn pipeline with multiple inputs/outputs
body: <p>How can I build a sklearn pipeline to do the following?</p>
  
  <p>What I have:</p>
  
  <pre><code>A, B = getAB(X_train)
  X_train = transform(X_train)
  model(A, B, X_train)
  </code></pre>
  
  <p>What I want:</p>
  
  <pre><code>pipe = Pipeline([
  (‘ab’, getAB),
  (‘tranf’, transform),
  (‘net’, net)
  ]
  pipe.fit(X_train, y_train)
  </code></pre>
  
  <p>Please help!</p>