Indexing bug (#233)

* fix

* test

python/src/indexing.cpp

@@ -120,6 +120,11 @@ array mlx_gather_nd(
     if (py::isinstance<py::slice>(idx)) {
       int start, end, stride;
       get_slice_params(start, end, stride, idx, src.shape(i));
+
+      // Handle negative indices
+      start = (start < 0) ? start + src.shape(i) : start;
+      end = (end < 0) ? end + src.shape(i) : end;
+
       gather_indices.push_back(arange(start, end, stride, uint32));
       num_slices++;
       is_slice[i] = true;