test(tensorflow): Fix interferometer gradient test

`tf.exp` was applied to a matrix, but it is an element-wise exponential
instead of a matrix exponential, hence the resulting matrix was not
unitary.

tests/backends/tensorflow/test_gradient.py

@@ -347,8 +347,7 @@ def test_Interferometer_fock_probabilities():
         )
 
         j = tf.complex(0.0, 1.0)
-        interferometer = tf.exp(j * hamiltonian)
-
+        interferometer = tf.linalg.expm(j * hamiltonian)
         with pq.Program() as program:
             pq.Q(all) | pq.StateVector((1, 0)) / np.sqrt(2)
             pq.Q(all) | pq.StateVector((1, 1)) / np.sqrt(2)
@@ -365,11 +364,11 @@ def test_Interferometer_fock_probabilities():
         jacobian,
         [
             [0.0, 0.0, 0.0, 0.0, 0.0, 0.0],
-            [0.0, 0.0, -0.42073548, -0.5950098, -0.90929735, -0.5950098],
-            [0.0, -0.42073548, 0.0, 0.0, -0.84147096, 0.0],
-            [0.0, -0.5950098, 0.0, 0.0, -1.1900196, 0.0],
-            [0.0, -0.90929735, -0.84147096, -1.1900196, -1.8185948, -1.1900196],
-            [0.0, -0.5950098, 0.0, 0.0, -1.1900196, 0.0],
+            [0.0, -0.4546487, 0.0, 0.0, -0.3162077, 0.0],
+            [0.0, 0.0, 0.4546488, -0.07391234, 0.0, -0.07391226],
+            [0.0, 0.0, -0.07391234, -0.3784013, 0.0, -0.3784013],
+            [0.0, -0.3162077, 0.0, 0.0, 0.75680256, 0.0],
+            [0.0, 0.0, -0.07391226, -0.3784013, 0.0, -0.37840137],
         ],
     )