Serializers deprecation (#5589)

This PR deprecates all serializers we plan to deprecate
* Deprecated GateOpSerializer, GateOpDeserializer, DeserializingArg, and SerializingArg using the class wrapper technique. 
* CircuitSerializer and serializer classes it depends on (`CircuitOp[De]Serializer`, `Serializer`) are left in place.
* Skipping deprecation of common serializers in `common_serializers.py` for now and remove them directly after the 0.15 release as it's very unlikely for users to access them directly. Most users interact with them via global gatesets, e.g.

https://github.com/quantumlib/Cirq/blob/abfc619bfe31e93573993c76cecfddc5689dea9b/cirq-google/cirq_google/serialization/gate_sets.py#L43-L64

But if time allows, I'll try to deprecate common serializers as well.

* Added mentions of serializer deprecations in global gateset deprecation warnings.

@dstrain115

Author: verult

cirq-google/cirq_google/__init__.py

@@ -160,53 +160,25 @@
 
 _register_resolver(_class_resolver_dictionary)
 
+
+_SERIALIZABLE_GATESET_DEPRECATION_MESSAGE = (
+    'SerializableGateSet and associated classes (GateOpSerializer, GateOpDeserializer,'
+    ' SerializingArgs, DeserializingArgs) will no longer be supported.'
+    ' In cirq_google.GridDevice, the new representation of Google devices, the gateset of a device'
+    ' is represented as a cirq.Gateset and is available as'
+    ' GridDevice.metadata.gateset.'
+    ' Engine methods no longer require gate sets to be passed in.'
+    ' In addition, circuit serialization is replaced by cirq_google.CircuitSerializer.'
+)
+
+
 _compat.deprecate_attributes(
     __name__,
     {
-        'XMON': (
-            'v0.16',
-            'SerializableGateSet will no longer be supported.'
-            ' In cirq_google.GridDevice, the new representation of Google devices, the gateset of '
-            ' a device is represented as a cirq.Gateset and is available as'
-            ' GridDevice.metadata.gateset.'
-            ' Engine methods no longer require gate sets to be passed in.'
-            ' In addition, circuit serialization is replaced by cirq_google.CircuitSerializer.',
-        ),
-        'FSIM_GATESET': (
-            'v0.16',
-            'SerializableGateSet will no longer be supported.'
-            ' In cirq_google.GridDevice, the new representation of Google devices, the gateset of '
-            ' a device is represented as a cirq.Gateset and is available as'
-            ' GridDevice.metadata.gateset.'
-            ' Engine methods no longer require gate sets to be passed in.'
-            ' In addition, circuit serialization is replaced by cirq_google.CircuitSerializer.',
-        ),
-        'SQRT_ISWAP_GATESET': (
-            'v0.16',
-            'SerializableGateSet will no longer be supported.'
-            ' In cirq_google.GridDevice, the new representation of Google devices, the gateset of '
-            ' a device is represented as a cirq.Gateset and is available as'
-            ' GridDevice.metadata.gateset.'
-            ' Engine methods no longer require gate sets to be passed in.'
-            ' In addition, circuit serialization is replaced by cirq_google.CircuitSerializer.',
-        ),
-        'SYC_GATESET': (
-            'v0.16',
-            'SerializableGateSet will no longer be supported.'
-            ' In cirq_google.GridDevice, the new representation of Google devices, the gateset of '
-            ' a device is represented as a cirq.Gateset and is available as'
-            ' GridDevice.metadata.gateset.'
-            ' Engine methods no longer require gate sets to be passed in.'
-            ' In addition, circuit serialization is replaced by cirq_google.CircuitSerializer.',
-        ),
-        'NAMED_GATESETS': (
-            'v0.16',
-            'SerializableGateSet will no longer be supported.'
-            ' In cirq_google.GridDevice, the new representation of Google devices, the gateset of '
-            ' a device is represented as a cirq.Gateset and is available as'
-            ' GridDevice.metadata.gateset.'
-            ' Engine methods no longer require gate sets to be passed in.'
-            ' In addition, circuit serialization is replaced by cirq_google.CircuitSerializer.',
-        ),
+        'XMON': ('v0.16', _SERIALIZABLE_GATESET_DEPRECATION_MESSAGE),
+        'FSIM_GATESET': ('v0.16', _SERIALIZABLE_GATESET_DEPRECATION_MESSAGE),
+        'SQRT_ISWAP_GATESET': ('v0.16', _SERIALIZABLE_GATESET_DEPRECATION_MESSAGE),
+        'SYC_GATESET': ('v0.16', _SERIALIZABLE_GATESET_DEPRECATION_MESSAGE),
+        'NAMED_GATESETS': ('v0.16', _SERIALIZABLE_GATESET_DEPRECATION_MESSAGE),
     },
 )

cirq-google/cirq_google/devices/known_devices.py

@@ -182,7 +182,7 @@ def create_device_proto_for_qubits(
                 gate = gs_proto.valid_gates.add()
                 gate.id = gate_id
 
-                if not isinstance(serializer, op_serializer.GateOpSerializer):
+                if not isinstance(serializer, op_serializer._GateOpSerializer):
                     # This implies that 'serializer' handles non-gate ops,
                     # such as CircuitOperations. No other properties apply.
                     continue

cirq-google/cirq_google/devices/serializable_device_test.py

@@ -27,7 +27,11 @@
 
 
 def _just_cz():
-    with cirq.testing.assert_deprecated('SerializableGateSet', deadline='v0.16', count=None):
+    # Deprecations: cirq_google.SerializableGateSet, cirq_google.GateOpSerializer, and
+    # cirq_google.GateOpDeserializer
+    with cirq.testing.assert_deprecated(
+        'SerializableGateSet', 'CircuitSerializer', deadline='v0.16', count=None
+    ):
         return cg.SerializableGateSet(
             gate_set_name='cz_gate_set',
             serializers=[
@@ -42,7 +46,11 @@ def _just_cz():
 
 
 def _just_meas():
-    with cirq.testing.assert_deprecated('SerializableGateSet', deadline='v0.16', count=None):
+    # Deprecations: cirq_google.SerializableGateSet, cirq_google.GateOpSerializer, and
+    # cirq_google.GateOpDeserializer
+    with cirq.testing.assert_deprecated(
+        'SerializableGateSet', 'CircuitSerializer', deadline='v0.16', count=None
+    ):
         return cg.SerializableGateSet(
             gate_set_name='meas_gate_set',
             serializers=[