The best answer is that you probably don't want to use Futhark for that - it's a specialised, rather restricted language. There are ways to encode irregular arrays, but it's not even remotely as ergonomic as in other languages (it can be plenty fast, however). For a particularly simple example, see https://futhark-lang.org/examples/triangular.html

In the future, or in a future similar language, one might well imagine an ability to "box" arrays, like in APL, which would allow true "arrays of arrays" with no restrictions on sizes.