Partial Support for tags in cirq_google protos (#7044)

* Partial Support for tags in cirq_google protos

- This adds several tags to cirq_google protos, and also adds
partial support for serialization and deserialization of tags.

Note: still pending are arguments for InternalTag objects and
better ability to serialize/deserialize internal tags that are
not public.

* Update cirq-google/cirq_google/ops/fsim_via_model_tag.py

Co-authored-by: Pavol Juhas <pavol.juhas@gmail.com>

* Update cirq-google/cirq_google/ops/physical_z_tag.py

Co-authored-by: Pavol Juhas <pavol.juhas@gmail.com>

* Update cirq-google/cirq_google/ops/internal_tag.py

Co-authored-by: Pavol Juhas <pavol.juhas@gmail.com>

* Update cirq-google/cirq_google/serialization/circuit_serializer.py

Co-authored-by: Pavol Juhas <pavol.juhas@gmail.com>

* Address some comments.

* Address other comment.

* Solve all the CI problems.

---------

Co-authored-by: Pavol Juhas <pavol.juhas@gmail.com>

Author: dstrain115

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

@@ -48,6 +48,9 @@ message Constant {
 
     // Operations used multiple times in a circuit
     Operation operation_value = 5;
+
+    // Tags used multiple times in a circuit
+    Tag tag_value = 6;
   }
 }
 
@@ -268,19 +271,110 @@ message Operation {
     int32 token_constant_index = 5;
   }
 
+  // To be deprecated
   repeated Tag tags = 22;
+
+  // Indices in the constant table for tags associated with the operation
+  repeated int32 tag_indices = 23;
 }
 
 message DynamicalDecouplingTag {
   optional string protocol = 1;
 }
 
+
+// Messages for tags that can modify operations
+// These are often directives to hardware specifying
+// how the operation should be executed.
 message Tag {
   oneof tag {
+    // Tag to denote a composite dynamical decoupling operation.
+    // Should generally be applied to cirq.I gates.
     DynamicalDecouplingTag dynamical_decoupling = 1;
+
+    // Directs the hardware to ignore moment-based
+    // synchronization and to instead schedule
+    // operations as soon as possible for these qubits.
+    NoSyncTag no_sync = 2;
+
+    // Operation should do phase matching to match phase
+    // required for subsequent operations
+    PhaseMatchTag phase_match = 3;
+
+    // Only applicable to Z gates and other gates
+    // that have Z gates as part of their internal operation
+    // This indicates that the Z gates could be executed on
+    // hardware rather than be computed virtually as part
+    // of phase matching.
+    PhysicalZTag physical_z = 4;
+
+    // Indicates that the operations are classical states
+    ClassicalStateTag classical_state = 5;
+
+    // Field id 6 Reserved for OverlayTag, will add in a subsequent PR.
+
+    // Uses parameter model to interpolate FSim gate.
+    FSimViaModelTag fsim_via_model = 7;
+
+    // Catch-all for all gates that do not fit into the
+    // above tags.
+    InternalTag internal_tag = 8;
   }
 }
 
+
+// Tag for operations that should do phase matching to match phase
+// required for subsequent operations
+message PhaseMatchTag {
+}
+
+message PhysicalZTag {
+}
+
+// Tag to indicate that a state prep circuit produces a classical state.
+// This serves as a hint to the compiler that we can ignore virtual Z phases
+message ClassicalStateTag {
+}
+
+
+message FSimViaModelTag {
+}
+
+// Tag to remove moment-based synchronization
+// The reverse and forward arguments control the
+// number of moments to reverse synchronization.
+message NoSyncTag {
+    oneof rev {
+        // Number of synchronizations before the operation to remove
+        int32 reverse = 1;
+
+        // Remove all possible synchronizations
+        bool remove_all_syncs_before = 2;
+    }
+    oneof fwd {
+        // Number of synchronizations after the operation to remove
+        int32 forward = 3;
+
+        // Remove all possible synchronizations
+        bool remove_all_syncs_after = 4;
+    }
+}
+
+
+// Tag to represent any internal tags or tags not yet
+// implemented in the proto.
+message InternalTag {
+  // Name of the tag
+  string tag_name = 1;
+  // Python package of the Tag
+  string tag_package = 2;
+
+  // Instantiation arguments of the Tag
+  map<string, Arg> tag_args = 3;
+  map<string, CustomArg> custom_args = 4;
+}
+
+
 // The instruction identifying the action taken on the quantum computer.
 message Gate {
   // Name for the Gate.