Adds mjlab.tasks import for env registration

[louislelay]

While trying to run this command uv run --extra rl scripts/velocity/rl/train.py --task Velocity-Flat-Unitree-Go1-v0, I ran into the following error:

spring@pal-springbasestation:~/mjlab$ uv run --extra rl scripts/velocity/rl/train.py --task Velocity-Flat-Unitree-Go1-v0
Traceback (most recent call last):
  File "/home/spring/mjlab/scripts/velocity/rl/train.py", line 104, in <module>
    tyro.cli(main)
    ~~~~~~~~^^^^^^
  File "/home/spring/mjlab/.venv/lib/python3.13/site-packages/tyro/_cli.py", line 230, in cli
    return run_with_args_from_cli()
  File "/home/spring/mjlab/scripts/velocity/rl/train.py", line 39, in main
    env_cfg = load_cfg_from_registry(task, "env_cfg_entry_point")
  File "/home/spring/mjlab/src/mjlab/third_party/isaaclab/isaaclab_tasks/utils/parse_cfg.py", line 54, in load_cfg_from_registry
    cfg_entry_point = gym.spec(task_name.split(":")[-1]).kwargs.get(entry_point_key)
                      ~~~~~~~~^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/home/spring/mjlab/.venv/lib/python3.13/site-packages/gymnasium/envs/registration.py", line 1000, in spec
    _check_version_exists(ns, name, version)
    ~~~~~~~~~~~~~~~~~~~~~^^^^^^^^^^^^^^^^^^^
  File "/home/spring/mjlab/.venv/lib/python3.13/site-packages/gymnasium/envs/registration.py", line 392, in _check_version_exists
    _check_name_exists(ns, name)
    ~~~~~~~~~~~~~~~~~~^^^^^^^^^^
  File "/home/spring/mjlab/.venv/lib/python3.13/site-packages/gymnasium/envs/registration.py", line 369, in _check_name_exists
    raise error.NameNotFound(
        f"Environment `{name}` doesn't exist{namespace_msg}.{suggestion_msg}"
    )
gymnasium.error.NameNotFound: Environment `Velocity-Flat-Unitree-Go1` doesn't exist.

The issue was caused by mjlab.tasks not being imported, so I added it to both the RL play and train scripts. I think it may have been removed by pre-commit, so I added # noqa: F401 to ensure it is kept.

[kevinzakka]

This does not happen in the tracking train and play scripts because we import the config from the task and do type casting of the returned env_cfg: env_cfg = cast(TrackingEnvCfg, load_cfg_from_registry(task, "env_cfg_entry_point")). I have a local branch that cleans up the velocity task and I'll push it later today but when I do I believe this won't be necessary. Sound good to you?

PS: It's great to see you already contributing to mjlab!

[louislelay]

oh ok, it makes sense! Sounds good to me, I'll close it then!

And yes, I wanted to begin trying it out today I've a few other things in mind to add but that's really great work congrats!