`reduce` with function

[AntonOresten]

There should be an easier way to do the equivalent of mean(abs2, x, dims=1) with the Einops syntax. Currently, one might have to do:

reduce((x; dims) -> mean(abs2, x; dims), x, (:a, ..) --> (..))

I'd consider moving the both the function used to reduce, and the function applied to each term, to the back of the reduce function like so:

reduce(x, (:a, ..) --> (..), mean, abs2)

Ignoring e.g. abs2, this is also closer to the Python syntax:

reduce(x, 'a ... -> ...', 'mean')