vela: Change Shape4D mutability usage

 - Removed requirement for cloning shapes when unique values required
   by forcing top-level immutability. This alleviates issues with Shapes
   being unintentionally shared and then mutated as if value-types.
 - Shape4D fields can no longer be assigned without replication.

Signed-off-by: Tim Hall <tim.hall@arm.com>
Change-Id: Ic0dbfa349eb0215eabefb4f4e2cf99f12d83699c
diff --git a/ethosu/vela/pass_packing.py b/ethosu/vela/pass_packing.py
index c973b9c..abd235f 100644
--- a/ethosu/vela/pass_packing.py
+++ b/ethosu/vela/pass_packing.py
@@ -231,7 +231,7 @@
                 ofm_tensor = op.ofm
                 if ofm_tensor is None:
                     ofm_tensor = op.outputs[0]
-                ofm_shape = op.ofm_shapes[0].clone() if op.run_on_npu else None
+                ofm_shape = op.ofm_shapes[0] if op.run_on_npu else None
 
                 build_pass((op,), ofm_tensor, ofm_shape)