fix fancy slicing in Nx

[Arsalaan-Alam]

This PR fixes #151

Updated slicing logic in nx/lib/core/frontend.ml:

• Only skip work when index lists are in true identity order: [0; 1; 2; …; n-1]
• For any other case (reordering, duplicates), dispatch to gather so the result matches expected semantics.

Added the following tests in nx/test/test_nx_indexing.ml:

• test_index_idx_reorder_rows
• test_index_idx_duplicate_rows

These tests ensure correct behavior for both permutations and repeated indices.

[ghennequin]

Thanks! There should be a more efficient way of checking whether or not indices is the [0;1;...n-1] range (your solution allocates a whole [0; 1; ...; n-1] range of indices, and then tests full equality with the provided indices; this introduces quite a bit of overhead). Can I suggest this instead:

let is_range n indices =
  let rec traverse id = function
    | [] -> id=n
    | hd :: tl when hd = id -> traverse (succ id) tl
    | _ -> false
  in
  traverse 0 indices

E.g.
is_range 3 [0;1;2] → true
is_range 3 [1;2;3] → false
is_range 3 [0;2;1] → false
...etc.

[Arsalaan-Alam]

you're right! thanks for pointing this out, updated the implementation

[ghennequin]

looks good to me, thanks @Arsalaan-Alam !