How to freeze parameters with `nnx` and `optax`?

I would like to know what is the best way to freeze parameters in a model using `nnx` and `optax` (https://flax.readthedocs.io/en/latest/guides/training_techniques/transfer_learning.html#optax-multi-transform).

I think it would be useful to add an example to https://flax.readthedocs.io/en/latest/nnx/index.html.