`uniform` corresponds to the earlier default implementation in master. For example in master,
 
    y_true = np.array([1.2])
    y_pred = np.array([2.3])
    mean_absolute_error(y_true, y_pred)
    1.0999999999999999

In this branch, 
    y_true = np.array([1.2])
    y_pred = np.array([2.3])
    mean_absolute_error(y_true, y_pred, output_weights='uniform')
    1.0999999999999999
    mean_absolute_error(y_true, y_pred, output_weights=None)
    array([ 1.1])

r2_score already raises this error, 
    ValueError: r2_score can only be computed given more than one sample.