Why does LSTMCell keep rngs in its state?

It seems that LSTMCell keeps rngs in its state:
https://github.com/google/flax/blob/a8a192ff167f8b25b9b568cfece44ef043a82dad/flax/nnx/nn/recurrent.py#L137

Is this intentional? Why?

I stumbled upon this because my recipe for checkpointing breaks when my model contains an LSTM:

```
import orbax.checkpoint as ocp
def savemodel(model, path):
_, state = nnx.split(model)
checkpointer = ocp.StandardCheckpointer()
checkpointer.save(path, state)
```

Calling `savemodel(model, path)` throws:
```
TypeError: JAX array with PRNGKey dtype cannot be converted to a NumPy array. Use jax.random.key_data(arr) if you wish to extract the underlying integer array.
```

This was surprising because I've been using that recipe before and never had a problem while using other non-LSTM modules.