message: Add SavedModel to custom gradients documentation.
diff: * `tf.saved_model`:
  * SavedModels can now save custom gradients. Use the option
  `tf.saved_model.SaveOption(experimental_custom_gradients=True)` to
  - enable this feature.
  + enable this feature. The documentation in [Advanced autodiff]
  + (https://www.tensorflow.org/guide/advanced_autodiff#custom_gradients)
  + has been updated.
  * TF Core:
  * Added `tf.config.experimental.reset_memory_stats` to reset the tracked
  