transforms.py: Avoid setting the barrier to be a uuid object (#625)

* transforms.py: Convert the barrier uuid label into a string

More info at Qiskit/qiskit#12581

* Fix `isinstance(label, UUID)` check

circuit_knitting/utils/transforms.py

@@ -27,7 +27,7 @@
 """
 from __future__ import annotations
 
-from uuid import uuid4, UUID
+from uuid import uuid4
 from collections import defaultdict
 from collections.abc import Sequence, Iterable, Hashable, MutableMapping
 from typing import NamedTuple, Callable
@@ -276,7 +276,7 @@ def _split_barriers(circuit: QuantumCircuit):
         num_qubits = len(inst.qubits)
         if num_qubits == 1 or inst.operation.name != "barrier":
             continue
-        barrier_uuid = uuid4()
+        barrier_uuid = f"_uuid={uuid4()}"
 
         # Replace the N-qubit barrier with a single-qubit barrier
         circuit.data[i] = CircuitInstruction(
@@ -298,13 +298,13 @@ def _combine_barriers(circuit: QuantumCircuit):
     uuid_map = defaultdict(list)
     for i, inst in enumerate(circuit):
         if (
-            inst.operation.name != "barrier"
-            or len(inst.qubits) != 1
-            or not isinstance(inst.operation.label, UUID)
+            inst.operation.name == "barrier"
+            and len(inst.qubits) == 1
+            and inst.operation.label is not None
+            and inst.operation.label.startswith("_uuid=")
         ):
-            continue
-        barrier_uuid = inst.operation.label
-        uuid_map[barrier_uuid].append(i)
+            barrier_uuid = inst.operation.label
+            uuid_map[barrier_uuid].append(i)
 
     # Replace the first single-qubit barrier in each group with the full-sized barrier
     cleanup_inst = []