Replace indexing with tf.gather (#710)

Author: Ashley Scillitoe

alibi_detect/cd/tensorflow/mmd_online.py

@@ -167,7 +167,7 @@ def _configure_thresholds(self):
 
     def _update_state(self, x_t: np.ndarray):  # type: ignore[override]
         self.t += 1
-        kernel_col = self.kernel(self.x_ref[self.ref_inds], x_t)
+        kernel_col = self.kernel(tf.gather(self.x_ref, self.ref_inds), x_t)
         self.test_window = tf.concat([self.test_window[(1 - self.window_size):], x_t], axis=0)
         self.k_xy = tf.concat([self.k_xy[:, (1 - self.window_size):], kernel_col], axis=1)