One fit_transform call on the pipeline may result in multiple fit_transform calls on some steps

[grzes314]

Consider 3 steps A, B, C, connected like this: A -> B, B -> C, A -> C. Say, we fit this pipeline by calling C.fit_transform(...). If A is initialized with force_fitting option, then its fit_transform method will be called twice, which is undesirable behavior. Even when force_fitting is False, transform is going to be called twice, which might require reconsidaration.