Open
Description
#15557 introduced a change in the GP module to allow a list of generic objects and kernels on those data, which is nice, but it violates our API AFAIK.
If we're going to allow that, there are many other places we can/should allow that as well (SVMs for instance).
But that doesn't really play well with the rest of the library (pipeline, GS, etc).
ping @scikit-learn/core-devs