fix(mlx): forward boundary-partitioned getitem instead of indexing a Slot (#21441)

## Summary

The MLX partitioner can place an `operator.getitem` inside a partition
while leaving its multi-output parent (e.g. `aten.native_layer_norm`)
outside it. In that case `_getitem_handler` receives the
already-selected element as a single boundary-input `Slot` rather than
the source tuple, so `a[idx]` raises `'Slot' object is not
subscriptable` and aborts the export.

This fix forwards the `Slot` directly when the resolved arg isn't a
tuple/list — the partitioner has already fed the selected element as the
partition input.

## Impact

Unblocks transformer / layer-norm-heavy models (e.g. RF-DETR) through
the MLX delegate. Localized, one-file change to the op handler; no
schema/serialization change.

## Test plan

- Verified that DETR-style exports (dozens of `native_layer_norm` ops)
lower and run correctly on device through the MLX delegate with this
change.
- Non-boundary getitem (source is a tuple/list of slots) is unchanged:
`a[idx]` still selects the element.


cc @metascroy

Co-authored-by: Scott Roy <161522778+metascroy@users.noreply.github.com>

Author: msluszniak

backends/mlx/ops.py

@@ -2272,9 +2272,13 @@ def _getitem_handler(P: MLXProgramBuilder, n: Node) -> Slot:
     require_kwargs(P.kwargs(n), set(), "operator.getitem")
     a, idx = args
     out = P.make_or_get_slot(n)
+    # When the multi-output parent is left outside the partition, the partitioner
+    # feeds this getitem the already-selected element as a single boundary-input
+    # Slot (not the source tuple). In that case forward it directly.
+    src = a[idx] if isinstance(a, (tuple, list)) else a
     P.emit(
         IdCopyNode(
-            x=P.slot_to_tid(a[idx]),
+            x=P.slot_to_tid(src),
             out=P.slot_to_tid(out),
         )
     )