Replace OpTree with OP_TREE (#6960)

There was a comment in `op_tree.py` to do this once mypy supports
recursive types. The referenced issue
python/mypy#731 is now closed, and the
suggested replacement seems to work.

<img width="643" alt="image"
src="https://github.com/user-attachments/assets/7333a667-773c-4557-a7b7-60770b21c0a4"
/>

cc @maffoo

Author: daxfohl

cirq-core/cirq/ops/op_tree.py

@@ -16,7 +16,6 @@
 """
 
 from typing import Callable, Iterable, Iterator, NoReturn, Union, TYPE_CHECKING
-from typing_extensions import Protocol
 
 from cirq._doc import document
 from cirq._import import LazyLoader
@@ -28,32 +27,7 @@
 moment = LazyLoader("moment", globals(), "cirq.circuits.moment")
 
 
-class OpTree(Protocol):
-    """The recursive type consumed by circuit builder methods.
-
-    An OpTree is a type protocol, satisfied by anything that can be recursively
-    flattened into Operations. We also define the Union type OP_TREE which
-    can be an OpTree or just a single Operation.
-
-    For example:
-    - An Operation is an OP_TREE all by itself.
-    - A list of operations is an OP_TREE.
-    - A list of tuples of operations is an OP_TREE.
-    - A list with a mix of operations and lists of operations is an OP_TREE.
-    - A generator yielding operations is an OP_TREE.
-
-    Note: once mypy supports recursive types this could be defined as an alias:
-
-    OP_TREE = Union[Operation, Iterable['OP_TREE']]
-
-    See: https://github.com/python/mypy/issues/731
-    """
-
-    def __iter__(self) -> Iterator[Union[Operation, 'OpTree']]:
-        pass
-
-
-OP_TREE = Union[Operation, OpTree]
+OP_TREE = Union[Operation, Iterable['OP_TREE']]
 document(
     OP_TREE,
     """An operation or nested collections of operations.

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

@@ -19,7 +19,7 @@
 Synthesis of Quantum Logic Circuits. Tech. rep. 2006,
 https://arxiv.org/abs/quant-ph/0406176
 """
-from typing import List, Callable, TYPE_CHECKING
+from typing import Callable, Iterable, List, TYPE_CHECKING
 
 from scipy.linalg import cossin
 
@@ -38,12 +38,11 @@
 
 if TYPE_CHECKING:
     import cirq
-    from cirq.ops import op_tree
 
 
 def quantum_shannon_decomposition(
     qubits: 'List[cirq.Qid]', u: np.ndarray, atol: float = 1e-8
-) -> 'op_tree.OpTree':
+) -> Iterable['cirq.Operation']:
     """Decomposes n-qubit unitary 1-q, 2-q and GlobalPhase gates, preserving global phase.
 
     The gates used are CX/YPow/ZPow/CNOT/GlobalPhase/CZ/PhasedXZGate/PhasedXPowGate.
@@ -141,7 +140,7 @@ def quantum_shannon_decomposition(
     yield from _msb_demuxer(qubits, u1, u2)
 
 
-def _single_qubit_decomposition(qubit: 'cirq.Qid', u: np.ndarray) -> 'op_tree.OpTree':
+def _single_qubit_decomposition(qubit: 'cirq.Qid', u: np.ndarray) -> Iterable['cirq.Operation']:
     """Decomposes single-qubit gate, and returns list of operations, keeping phase invariant.
 
     Args:
@@ -186,7 +185,7 @@ def _single_qubit_decomposition(qubit: 'cirq.Qid', u: np.ndarray) -> 'op_tree.Op
 
 def _msb_demuxer(
     demux_qubits: 'List[cirq.Qid]', u1: np.ndarray, u2: np.ndarray
-) -> 'op_tree.OpTree':
+) -> Iterable['cirq.Operation']:
     """Demultiplexes a unitary matrix that is multiplexed in its most-significant-qubit.
 
     Decomposition structure:
@@ -249,7 +248,7 @@ def _nth_gray(n: int) -> int:
 
 def _multiplexed_cossin(
     cossin_qubits: 'List[cirq.Qid]', angles: List[float], rot_func: Callable = ops.ry
-) -> 'op_tree.OpTree':
+) -> Iterable['cirq.Operation']:
     """Performs a multiplexed rotation over all qubits in this unitary matrix,
 
     Uses ry and rz multiplexing for quantum shannon decomposition