Make device parameter units/idx optional (#6264)

- Currently, the device parameters are picking
up zero as the index and failing.
- Need to make them optional so we can set this to
None when it is not an array index.
- Yay for proto3!

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

@@ -90,11 +90,11 @@ message DeviceParameter {
     repeated string path = 1;
 
     // If the value is an array, the index of the array to change.
-    int64 idx = 2;
+    optional int64 idx = 2;
 
     // String representation of the units, if any.
     // Examples: "GHz", "ns", etc.
-    string units = 3;
+    optional string units = 3;
 
     // Note that the device parameter values will be populated
     // by the sweep values themselves.

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

@@ -105,7 +105,13 @@ def sweep_from_proto(msg: run_context_pb2.Sweep) -> cirq.Sweep:
         key = msg.single_sweep.parameter_key
         if msg.single_sweep.HasField("parameter"):
             metadata = DeviceParameter(
-                path=msg.single_sweep.parameter.path, idx=msg.single_sweep.parameter.idx
+                path=msg.single_sweep.parameter.path,
+                idx=msg.single_sweep.parameter.idx
+                if msg.single_sweep.parameter.HasField("idx")
+                else None,
+                units=msg.single_sweep.parameter.units
+                if msg.single_sweep.parameter.HasField("units")
+                else None,
             )
         else:
             metadata = None

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

@@ -52,6 +52,11 @@ def _values(self) -> Iterator[float]:
             [1, 1.5, 2, 2.5, 3],
             metadata=DeviceParameter(path=['path', 'to', 'parameter'], idx=2, units='GHz'),
         ),
+        cirq.Points(
+            'b',
+            [1, 1.5, 2, 2.5, 3],
+            metadata=DeviceParameter(path=['path', 'to', 'parameter'], idx=None),
+        ),
         cirq.Linspace('a', 0, 1, 5) * cirq.Linspace('b', 0, 1, 5),
         cirq.Points('a', [1, 2, 3]) + cirq.Linspace('b', 0, 1, 3),
         (
@@ -69,6 +74,8 @@ def test_sweep_to_proto_roundtrip(sweep):
     msg = v2.sweep_to_proto(sweep)
     deserialized = v2.sweep_from_proto(msg)
     assert deserialized == sweep
+    # Check that metadata is the same, if it exists.
+    assert getattr(deserialized, 'metadata', None) == getattr(sweep, 'metadata', None)
 
 
 def test_sweep_to_proto_linspace():