code: def invert_features(features):
    
    inverted_features = collections.defaultdict(list)
    for transform in six.itervalues(features):
      source_column = transform['source_column']
      inverted_features[source_column].append(transform)
  
    return dict(inverted_features)
docs: Make a dict in the form source column : set of transforms.
  
    Note that the key transform is removed.