Add DM Control Quadruped

[yardenas]

Hi ant thanks again for the amazing work!

I have a hacky implementation of the deepmind control quadruped environment, currently only with the Run and Walk tasks.

I am happy to contribute a cleaner version of it to mujoco_playground, if that would be useful to anyone. Please let me know!

[btaba]

Hi @yardenas thanks for the note , feel free to contribute a new env to the dm control suite fork!

[yardenas]

@btaba sounds good! I'm on it. It would probably take me a few weeks to start working on it and make the code cleaner. Hope to update you soon!

[yardenas]

@btaba, two questions about the design of the environment:

1. Currently we do not sample the initial position of the quadruped as done in the original implementation as it slows down training (I think ~5 additional minutes). What's your opinion about that? I could try to optimize my implementation of _find_non_contacting_height, but not sure exactly if we can gain a lot.
2. I disabled some contacts of the knees with the ground to speed up performance. What do you think about that?