[Bug Fix] Small fixes for new mujoco release 3.0.0

[rodrigodelazcano]

Description

• Fix for mujoco rendering: The MjData member solver_iter has been renamed to solver_niter as specified in the changelog. We are also not using islands in our environments so we just output the first element of the list. Fixes MjData has no attribute solver_iter google-deepmind/mujoco#1107

• Fix for swimmer xml: As specified in the 11 point of the changelog the collision argument has been removed from option.
  This PR makes the changes specified in the migration guide. The changelog provides the migration guide for "pair" that in the previous mujoco release documentation appears to be named as "predefined" such as in the swimmer xml (seems like a release note errata). I haven't compared reproducibility with the current Swimmer-v5 environment, but as specified in the docs mujoco only guaranties reproducibility for a fixed version https://mujoco.readthedocs.io/en/3.0.0/computation/index.html?highlight=reproducibility#reproducibility

Type of change

Please delete options that are not relevant.

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• This change requires a documentation update

Screenshots

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Checklist:

• I have run the pre-commit checks with pre-commit run --all-files (see CONTRIBUTING.md instructions to set it up)
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• My changes generate no new warnings
• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes

[rodrigodelazcano]

This will also require a version bump of mujoco to >=3.0 right?

[Kallinteris-Andreas]

We should be careful with changing the models, that would require a lot of validation

[pseudo-rnd-thoughts]

We should be careful with changing the models, that would require a lot of validation

Is one of you able to gather training results for the updated swimmer to compare to the old?

[Kallinteris-Andreas]

@pseudo-rnd-thoughts working on that

[rodrigodelazcano]

Thank you @Kallinteris-Andreas for testing. Just to add to the discussion, the xml changes don't affect the behavior/dynamics of the model. Seems like mujoco made this change because it was redundant to specify it this way. To explain how the changes don't modify the behavior:

• collision="predefined" made mujoco check for collisions/contacts only with geoms that are specified as pairs (https://mujoco.readthedocs.io/en/2.3.7/XMLreference.html?highlight=pair#contact-pair). The Swimmer xml doesn't have any geom pairs so no collisions are computed at all during simulation. This allows for the agent to slide without friction through the floor.
• mujoco==3.0.0 removes the collision="predefined" argument in favor of disabling non-pair dynamic collisions (determined via contype/conaffinity) by setting contype and conaffinity to 0 in all geoms of the model.

Training results should be identically hopefully.

[Kallinteris-Andreas]

I tested the new swimmer.xml with those versions of mujoco for 1mil steps each and have confirmed identical behavior

(validation repo: https://github.com/Kallinteris-Andreas/gym-mjc-v5-model-validation/tree/main/swimmer)

tested mujoco versions
"mujoco==2.1.5"
"mujoco==2.2.0"
"mujoco==2.2.1"
"mujoco==2.2.2"
"mujoco==2.3.0"
"mujoco==2.3.1"
"mujoco==2.3.2"
"mujoco==2.3.3"
"mujoco==2.3.5"
"mujoco==2.3.6"
"mujoco==2.3.7"
"mujoco-py==2.1.2.14" # note rendering was not identical, but that is unrelated to the model #690

[Kallinteris-Andreas]

It is possible to maintain support for both pre 3.0 and post 3.0 mujoco (which is important for reproducibility), by doing this

if mujoco.__version__ => "3.0.0":
    self.add_overlay(
        bottomleft, "Solver iterations", str(self.data.solver_niter[0] + 1)
    )
elif mujoco.__version__ < "3.0.0":
    self.add_overlay(
        bottomleft, "Solver iterations", str(self.data.solver_iter + 1)
    )

[pseudo-rnd-thoughts]

So we are good to merge with no impact on the end users