Support negative dimensions on "aten.split_with_sizes_copy.default" (#16006)

### Summary
Support a negative dimension for MPS SplitWithSizes implementation. 
### Test plan
I re-registered this node visitor on a model that previously failed to
export with the error message
```
RuntimeError: split_copy: dim -1 out of range for input tensor with 2 dimensions
```
and it succeeded.

Author: jgibson2

backends/apple/mps/operators/shape_ops.py

@@ -242,11 +242,14 @@ def define_node(
         output_ids = self.define_tensor_list(node, mps_graph)
         split_sizes = eval_shape(cast(torch.SymInt, node.args[1]))
         dim = cast(int, node.args[2])
+        orig_dim = dim
         input_shape = get_shape(get_input_node(node, 0))
+        if dim < 0:
+            dim += len(input_shape)
 
         if dim < 0 or dim >= len(input_shape):
             raise RuntimeError(
-                f"split_copy: dim {dim} out of range for input tensor with {len(input_shape)} dimensions"
+                f"split_copy: dim {orig_dim} out of range for input tensor with {len(input_shape)} dimensions"
             )
 
         mps_node = MPSNode(