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.