> See #2086, and note that the key must include precisely the parameters
> affecting steps prior to the current one being evaluated,

No: if X and y are used to compute the hash, you don't need to know where
they come from. This is actually a benefit of not trying to be clever:
changing 'n_jobs', or 'verbose' on previous transforms will trigger a
recompute down the line only if the transform is actually changed.