Allow loading variants for MuJoCo descriptions

[fabinsch]

Hi @stephane-caron ,

thanks for this nice library to load a bunch of robot models with different frameworks. Lately I was trying some mujoco models and I had a quick fix locally to load the scene.xml (which was more useful in my case).

I saw #74 and also #62 and thought that a minimal adaptation allowing for both could be the change proposed in this PR.
This adds a variant keyword, that I set the scene by default. If a robot has no scene.xml, it will still just load the pure robot model as define in each specific file per robot. With this solution, we do not have to modify every single file but I am very open to changes and happy to integrate your remarks.

[coveralls]

Pull Request Test Coverage Report for Build 8786290992

Details

• 6 of 15 (40.0%) changed or added relevant lines in 1 file are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage decreased (-0.9%) to 97.702%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
robot_descriptions/loaders/mujoco.py	6	15	40.0%

Totals
Change from base Build 8784910518:	-0.9%
Covered Lines:	978
Relevant Lines:	1001

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💛 - Coveralls

[stephane-caron]

Thank you for this feature that can be immediately useful to MuJoCo users 👍 I suggest we roll it in a backward-compatible manner with v1.10+, then have a more advanced variant mechanism for v2.

[kevinzakka]

Just chiming in that I disagree that scene should be the default.

[stephane-caron]

Just chiming in that I disagree that scene should be the default.

Yes, this was the original proposal but we have changed it in f609695. Now the PR is backward compatible and one should pass variant="scene" explicitly to load the scene.

[stephane-caron]

Thank you for this contribution @fabinsch 😃

[kevinzakka]

Thank you @fabinsch and @stephane-caron!