Add power function and test to backend

tensornetwork/backends/abstract_backend.py

@@ -988,3 +988,16 @@ def deserialize_tensor(self, s: str) -> Tensor:
     raise NotImplementedError(
         "Backend '{}' has not implemented deserialize_tensor.".format(
             self.name))
+
+  def power(self, a: Tensor, b: Tensor) -> Tensor:
+    """
+    Returns the element-wise exponentiation of tensor a raised to tensor b.
+
+    Args:
+      a: The tensor containing the bases.
+      b: The tensor containing the powers.
+
+    Returns:
+      The tensor that is each element of a raised to the corresponding power in b.
+    """
+    raise NotImplementedError(f"Backend {self.name} has not implemented power.")

tensornetwork/backends/backend_test.py

@@ -436,3 +436,8 @@ def test_pivot_not_implemented():
   backend = AbstractBackend()
   with pytest.raises(NotImplementedError):
     backend.pivot(np.ones((2, 2)))
+
+def test_power_not_implemented():
+  backend = AbstractBackend()
+  with pytest.raises(NotImplementedError):
+    backend.power(np.array([1, 2]), np.array([1, 2]))