Support MLXArray initialization from nested Swift Arrays

[awni]

This is mostly a convenience but it can be quite handy and we support it in Python:

let a = MLXArray([[7, 8], [9, 10]])

[davidkoski]

I need to play around with that. We can certainly do it for a fixed number of dimensions (we need an initializer for each number of dimensions). Maybe we can do something fancy with generics and gt an arbitrary number, but I think the types will be tricky.

[JimWallace]

MLXArray to nested Swift arrays would also be handy (I find that I'm re-writing this code a lot).

[davidkoski]

MLXArray to nested Swift arrays would also be handy (I find that I'm re-writing this code a lot).

How do you deal with the return type? asArray2() -> [[T]] and asArray3() -> [[[T]]]?

[JimWallace]

I’ve just been working with 2D matrices, and flipping between MLXArray and [[Float]], so pretty pedestrian but probably one of the most common use cases. Maybe I missed asArray2, is that already a thing?

[davidkoski]

No, not a thing -- just pondering how the types would work.

[JimWallace]

Oh I see! Yes, I’d imagine what you have would be useful for T: HasDType in very much the same way asArray works now, but maintaining the shape in Swift’s array.

I don’t know how to gracefully handle cases where shape doesn’t match, but I think it would be fair to throw an error much like the other precondition checks in MLX.