Add default decompositions for XXPowGate and YYPowGate which terminate in CZPowGate. (quantumlib#4862)

Part of quantumlib#4858

Decompositions are tested as part of `assert_eigengate_implements_consistent_protocols`, which is already tested for both gates.

Author: tanujkhattar

cirq/ops/parity_gates.py

@@ -104,6 +104,11 @@ def _decompose_into_clifford_with_qubits_(self, qubits):
             ]
         return NotImplemented
 
+    def _decompose_(self, qubits: Tuple['cirq.Qid', ...]) -> 'cirq.OP_TREE':
+        yield common_gates.YPowGate(exponent=-0.5).on_each(*qubits)
+        yield ZZPowGate(exponent=self.exponent, global_shift=self.global_shift)(*qubits)
+        yield common_gates.YPowGate(exponent=0.5).on_each(*qubits)
+
     def _circuit_diagram_info_(
         self, args: 'cirq.CircuitDiagramInfoArgs'
     ) -> Union[str, 'protocols.CircuitDiagramInfo']:
@@ -216,6 +221,11 @@ def _decompose_into_clifford_with_qubits_(self, qubits):
             ]
         return NotImplemented
 
+    def _decompose_(self, qubits: Tuple['cirq.Qid', ...]) -> 'cirq.OP_TREE':
+        yield common_gates.XPowGate(exponent=0.5).on_each(*qubits)
+        yield ZZPowGate(exponent=self.exponent, global_shift=self.global_shift)(*qubits)
+        yield common_gates.XPowGate(exponent=-0.5).on_each(*qubits)
+
     def _circuit_diagram_info_(
         self, args: 'cirq.CircuitDiagramInfoArgs'
     ) -> 'cirq.CircuitDiagramInfo':