Add units to device parameter (#6140)

* Add units to device parameter

- Add the ability to add unit type for a device parameter.
- This will be important for server-side deserialization, where the server will need to know what units the float values are in.
- This will allow us to do sweeps of parameters that have units, such as nanoseconds, or GigaHertz.

cirq-google/cirq_google/json_test_data/cirq.google.DeviceParameter.json

@@ -6,5 +6,6 @@
     "key"
   ],
   "idx": 4,
-  "value": 7.5
+  "value": 7.5,
+  "units": "GHz"
 }

cirq-google/cirq_google/json_test_data/cirq.google.DeviceParameter.repr

@@ -1 +1 @@
-cirq_google.study.device_parameter.DeviceParameter(path=['test', 'subdir', 'key'], idx=4, value=7.5)
+cirq_google.study.device_parameter.DeviceParameter(path=['test', 'subdir', 'key'], idx=4, value=7.5, units='GHz')

cirq-google/cirq_google/study/device_parameter.py

@@ -46,16 +46,19 @@ class DeviceParameter(SupportsDeviceParameter):
            entry in a list.
        idx: If this key is an array, which index to modify.
        value: value of the parameter to be set, if any.
+       units: string value of the unit type of the value, if any.
+          For instance, "GHz", "MHz", "ns", etc.
     """
 
     path: Sequence[str]
     idx: Optional[int] = None
     value: Optional[Any] = None
+    units: Optional[str] = None
 
     def __repr__(self) -> str:
         return (
             'cirq_google.study.DeviceParameter('
-            f'path={self.path!r}, idx={self.idx}, value={self.value!r})'
+            f'path={self.path!r}, idx={self.idx}, value={self.value!r}, units={self.units!r})'
         )
 
     @classmethod
@@ -64,7 +67,9 @@ def _json_namespace_(cls) -> str:
 
     @classmethod
     def _from_json_dict_(cls, path, idx, value, **kwargs):
-        return DeviceParameter(path=path, idx=idx, value=value)
+        return DeviceParameter(
+            path=path, idx=idx, value=value, units=kwargs['units'] if 'units' in kwargs else None
+        )
 
     def _json_dict_(self) -> Dict[str, Any]:
-        return cirq.obj_to_dict_helper(self, ["path", "idx", "value"])
+        return cirq.obj_to_dict_helper(self, ["path", "idx", "value", "units"])

cirq-google/cirq_google/study/device_parameter_test.py

@@ -16,5 +16,7 @@
 
 
 def test_parameters():
-    param = cirq_google.study.DeviceParameter(path=('test', 'subdir'), idx=2, value='tmp')
+    param = cirq_google.study.DeviceParameter(
+        path=('test', 'subdir'), idx=2, value='tmp', units='GHz'
+    )
     cirq.testing.assert_equivalent_repr(param, global_vals={'cirq_google': cirq_google})