Address review comments for PR #7390 (#7441)

There were two nits that I didn't address in the original PR and I do
here.

For the `for` loop calling `merged_ops.extend()`, I couldn't rewrite it
entirely as:
```
merged_ops.extend(_transform_single_qubit_operations_to_phased_x_and_z(qubit_ops, atol=atol) for qubit_ops in pending_ops.values())
```
because mypy complained about type mismatch: `merged_ops.extend` expects
an iterator over `Operation`, but
`_transform_single_qubit_operations_to_phased_x_and_z` returns a list of
operations, and so with the `for` loop it is an iterator over lists.

Co-authored-by: Noureldin <[email protected]>

Author: codrut3

cirq-core/cirq/transformers/analytical_decompositions/two_qubit_to_cz.py

@@ -226,12 +226,15 @@ def _merge_single_qubit_gates(
             transformation.
     Returns:
         new sequence of operations after merging gates
+
+    Raises:
+        ValueError: if one of the operations is not on 1 or 2 qubits
     """
     merged_ops: list[ops.Operation] = []
     pending_ops: dict[tuple[cirq.Qid, ...], list[ops.Operation]] = dict()
     for op in operations:
         if protocols.num_qubits(op) == 2:
-            for _, qubit_ops in pending_ops.items():
+            for qubit_ops in pending_ops.values():
                 merged_ops.extend(
                     _transform_single_qubit_operations_to_phased_x_and_z(qubit_ops, atol=atol)
                 )
@@ -242,8 +245,10 @@ def _merge_single_qubit_gates(
             if op.qubits not in pending_ops:
                 pending_ops[op.qubits] = []
             pending_ops[op.qubits].append(op)
+        else:
+            raise ValueError(f'operation is on {protocols.num_qubits(op)} qubits, expected 1 or 2')
     # Merge remaining pending operations
-    for _, qubit_ops in pending_ops.items():
+    for qubit_ops in pending_ops.values():
         merged_ops.extend(
             _transform_single_qubit_operations_to_phased_x_and_z(qubit_ops, atol=atol)
         )

cirq-core/cirq/transformers/analytical_decompositions/two_qubit_to_cz_test.py

@@ -26,6 +26,7 @@
 from cirq.transformers.analytical_decompositions.two_qubit_to_cz import (
     _is_trivial_angle,
     _parity_interaction,
+    cleanup_operations,
     two_qubit_matrix_to_diagonal_and_cz_operations,
 )
 
@@ -294,3 +295,11 @@ def test_remove_partial_czs_or_fail() -> None:
         _ = cirq.transformers.analytical_decompositions.two_qubit_to_cz._remove_partial_czs_or_fail(
             [CZ**-0.5], atol=1e-9
         )
+
+
+@pytest.mark.parametrize("gate", [cirq.CCZ, cirq.GlobalPhaseGate(1.0)])
+def test_cleanup_operations_raises_if_op_not_on_1_or_2_qubits(gate: cirq.Gate) -> None:
+    qubits = cirq.LineQubit.range(gate.num_qubits())
+    op = gate.on(*qubits)
+    with pytest.raises(ValueError, match="expected 1 or 2"):
+        cleanup_operations([op], atol=1e-8)