Serialize tags on moments in cirq_google protos (#7493)

- Tags were added to Moments this month.
- Adds the ability to serialize tags on Moments using cirq_google
protos.

Author: dstrain115

cirq-google/cirq_google/api/v2/program.proto

@@ -109,6 +109,9 @@ message Moment {
 
   // Deprecated field id, do not use.
   reserved 3;
+  
+  // Indices in the constant table for tags associated with the circuit
+  repeated int32 tag_indices = 5;
 }
 
 // The language in which the program is expressed.

cirq-google/cirq_google/api/v2/program_pb2.py

@@ -168,6 +168,11 @@ def _serialize_circuit(
                         raw_constants[op] = op_index
                         moment_proto.operation_indices.append(op_index)
 
+            for tag in moment.tags:
+                self._serialize_tag(
+                    tag, moment_proto, constants=constants, raw_constants=raw_constants
+                )
+
             # Add this moment to the constants table
             constants.append(v2.program_pb2.Constant(moment_value=moment_proto))
             moment_index = len(constants) - 1
@@ -314,7 +319,7 @@ def _serialize_gate_op(
     def _serialize_tag(
         self,
         tag: Hashable,
-        msg: v2.program_pb2.Operation | v2.program_pb2.Circuit,
+        msg: v2.program_pb2.Operation | v2.program_pb2.Moment | v2.program_pb2.Circuit,
         *,
         constants: list[v2.program_pb2.Constant],
         raw_constants: dict[Any, int],
@@ -502,6 +507,7 @@ def _deserialize_moment(
         deserialized_constants: list[Any],
     ) -> cirq.Moment:
         moment_ops = []
+        tags = []
         for op in moment_proto.operations:
             if self.op_deserializer and self.op_deserializer.can_deserialize_proto(op):
                 gate_op = self.op_deserializer.from_proto(
@@ -524,7 +530,10 @@ def _deserialize_moment(
             )
         for operation_index in moment_proto.operation_indices:
             moment_ops.append(deserialized_constants[operation_index])
-        moment = cirq.Moment(moment_ops)
+
+        for tag_index in moment_proto.tag_indices:
+            tags.append(deserialized_constants[tag_index])
+        moment = cirq.Moment(moment_ops, tags=tuple(tags))
         return moment
 
     def _deserialize_gate_op(

cirq-google/cirq_google/api/v2/program_pb2.pyi

@@ -1204,6 +1204,20 @@ def test_custom_tag_serializer_with_tags_outside_constants():
     assert serializer.deserialize(circuit_proto) == expected_circuit
 
 
+def test_moments_with_tags():
+    serializer = cg.CircuitSerializer(
+        tag_serializer=DiscountTagSerializer(), tag_deserializer=DiscountTagDeserializer()
+    )
+    original_circuit = cirq.Circuit(
+        cirq.Moment(cirq.X(cirq.GridQubit(1, 1))).with_tags(DiscountTag(0.50)),
+        cirq.Moment(cirq.Z(cirq.GridQubit(2, 2))).with_tags(cg.CalibrationTag("abc")),
+    )
+    deserialized_circuit = serializer.deserialize(serializer.serialize(original_circuit))
+    assert original_circuit == deserialized_circuit
+    assert deserialized_circuit[0].tags == (DiscountTag(0.50),)
+    assert deserialized_circuit[1].tags == (cg.CalibrationTag("abc"),)
+
+
 def test_reset_gate_with_improper_argument():
     serializer = cg.CircuitSerializer()