Demonstrate fast training using an off the shelf trainer and Brax Gym

[erikfrey]

We will probably use CleanRL for this: https://github.com/vwxyzjn/cleanrl

/cc @vwxyzjn

[erikfrey]

We have done this in a colab here:

Brax Training with PyTorch on GPU

We wrote a PPO trainer using vanilla PyTorch. It trains at 150,000 steps/second on a Colab GPU, but we suspect there is room for improvement - possibly by adding automatic mixed precision, or following other recommendations in the PyTorch performance tuning guide.