Add metadata property to device. (quantumlib#4868)

Adds metadata property default to None to all devices. I'm going to leave this open to vendors to add to this as they see fit. Part of quantumlib#4743 . Note that if in the process of deprecating other things for quantumlib#4744 , I will pull vendor devices onto using metadata to keep behavior working.

cirq/devices/device.py

@@ -105,6 +105,15 @@ def decompose_operation(self, operation: 'cirq.Operation') -> 'cirq.OP_TREE':
         """
         return operation
 
+    @property
+    def metadata(self) -> Optional['DeviceMetadata']:
+        """Returns the associated Metadata with the device if applicable.
+
+        Returns:
+            `cirq.DeviceMetadata` if specified by the device otherwise None.
+        """
+        return None
+
     def validate_operation(self, operation: 'cirq.Operation') -> None:
         """Raises an exception if an operation is not valid.
 

cirq/devices/device_test.py

@@ -78,6 +78,13 @@ def test_qid_pair():
         cirq.SymmetricalQidPair(q0, q0)
 
 
+def test_device_metadata():
+    class RawDevice(cirq.Device):
+        pass
+
+    assert RawDevice().metadata is None
+
+
 def test_metadata():
     qubits = cirq.LineQubit.range(4)
     graph = nx.star_graph(3)