Make grid_qubits addable (#2681)

- Fixes #2637

Author: dstrain115

cirq/devices/grid_qubit.py

@@ -163,6 +163,8 @@ def _json_dict_(self):
         return protocols.obj_to_dict_helper(self, ['row', 'col'])
 
     def __add__(self, other: Tuple[int, int]) -> 'GridQubit':
+        if isinstance(other, GridQubit):
+            return GridQubit(row=self.row + other.row, col=self.col + other.col)
         if not (isinstance(other, tuple) and len(other) == 2 and
                 all(isinstance(x, int) for x in other)):
             raise TypeError(
@@ -171,6 +173,8 @@ def __add__(self, other: Tuple[int, int]) -> 'GridQubit':
         return GridQubit(row=self.row + other[0], col=self.col + other[1])
 
     def __sub__(self, other: Tuple[int, int]) -> 'GridQubit':
+        if isinstance(other, GridQubit):
+            return GridQubit(row=self.row - other.row, col=self.col - other.col)
         if not (isinstance(other, tuple) and len(other) == 2 and
                 all(isinstance(x, int) for x in other)):
             raise TypeError(

cirq/devices/grid_qubit_test.py

@@ -154,6 +154,10 @@ def test_grid_qubit_add_subtract():
     assert (2, 5) + cirq.GridQubit(1, 2) == cirq.GridQubit(3, 7)
     assert (2, 5) - cirq.GridQubit(1, 2) == cirq.GridQubit(1, 3)
 
+    assert cirq.GridQubit(1, 2) + cirq.GridQubit(3, 5) == cirq.GridQubit(4, 7)
+    assert cirq.GridQubit(3, 5) - cirq.GridQubit(2, 1) == cirq.GridQubit(1, 4)
+    assert cirq.GridQubit(1, -2) + cirq.GridQubit(3, 5) == cirq.GridQubit(4, 3)
+
 
 def test_grid_qubit_neg():
     assert -cirq.GridQubit(1, 2) == cirq.GridQubit(-1, -2)