`nnx.vmap` use the same random key `rngs` inside nnx.Module across vectorization.

Hi,

If I run something like:
```
def inference_fn(model, x):
y = model(input)
return y

ys = nnx.vmap(inference_fn, in_axes=(None, 0))(model, xs)
```
the random key that is used across the vectorization is unique. It means that stochastic functions will have the same behavior across the batch.

- Is this the intended behavior for nnx.vmap when dealing with stochastic models?
- If yes, is there a recommended way to ensure independent random keys are used for each batch element when using nnx.vmap?

Thanks!