VAE example outdated

The VAE example in /flax/examples/vae seems to have some outdated requirements, jax.random usage, and absl.flags usage.

Modifications I made to get the example running:

- change jax.random.key to jax.random.PRNGKey
- install clu==0.0.10
- run main.py without workdir argument (change README to not use --workdir)

### System information
- jax==0.4.6
- flax==0.6.9
- other requirements installed from requirements.txt

I'd be happy to open a PR with these changes.