how to run mujoco variant environments

[esquires]

Greetings,

Thank you for putting together this repo. I was having difficulty recreating the images under the "Robust MuJoCo Variant Tasks" section of the README. Would you mind posting instructions on how to inject that environment noise? Thanks!

Here are a couple of the images as examples:

https://github.com/SafeRL-Lab/Robust-Gymnasium/blob/main/demos/robust_mujoco_dynamics/Ant_variant/Ant_variant_3%2000_00_00-00_00_30.gif

https://github.com/SafeRL-Lab/Robust-Gymnasium/blob/main/demos/robust_mujoco_dynamics/Ant_variant/Ant_variant_5%2000_00_00-00_00_30.gif