Possible generalization of gufunc keepdims for multiple outputs #11118

#11098 introduced a keepdims argument for gufuncs for which all the inputs had the same number of core dimensions and all the outputs had none; for those, it would insert dimensions of length 1 in the outputs, i.e., a signature like (i),(i)->(),() is changed to (i),(i)->(1),(1).

In #11098 (comment), @eric-wieser wondered whether it should also support signatures like (i),(i)->(),(i), i.e., allow outputs with the same number of dimensions as the inputs as well as those with no core dimensions. This issue is to remind us of this suggestion.