[Bug Report] JointAction not preserving order when using all joints

[T-K-233]

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Describe the bug

The mdp.JointPositionActionCfg in ActionsCfg allow us to specify the desired joint order by passing joint_names and setting preserve_order=True. However, this is not effective when we specify order for all joints.

Steps to reproduce

Using task "Isaac-Velocity-Flat-Unitree-A1-v0" as example,

when using the following config

@configclass
class ActionsCfg:
    """Action specifications for the MDP."""
    joint_pos = mdp.JointPositionActionCfg(
        asset_name="robot",
        joint_names=[
            "FL_hip_joint", "FL_thigh_joint", "FL_calf_joint",
            "FR_hip_joint", "FR_thigh_joint", "FR_calf_joint",
            "RL_hip_joint", "RL_thigh_joint", "RL_calf_joint", 
            "RR_hip_joint", "RR_thigh_joint", "RR_calf_joint"
        ],
        scale=0.5,
        use_default_offset=True,
        preserve_order=True
    )

we expect the joint orders to be [0, 4, 8, 1, 5, 9, 2, 6, 10, 3, 7, 11]. However, the _joint_ids that the JointAction sees is still a None slice (slice(None, None, None)).

This is due to this logic in JointAction

    def __init__(self, cfg: actions_cfg.JointActionCfg, env: ManagerBasedEnv) -> None:
        ...
        # Avoid indexing across all joints for efficiency
        if self._num_joints == self._asset.num_joints:
            self._joint_ids = slice(None)
        ...

It empties the _joint_ids even if we set preserve_order to be True. A suggested fix is to change the condition to be

self._num_joints == self._asset.num_joints and not self.cfg.preserve_order

which allow users to force a certain joint order of the policy output.

System Info

• Commit: dd044a4
• Isaac Sim Version: 4.2
• OS: Ubuntu 24.04
• GPU: RTX 4070 Ti Super
• CUDA: 12.4
• GPU Driver: 550.120

Additional context

None

Checklist

• I have checked that there is no similar issue in the repo (required)
• I have checked that the issue is not in running Isaac Sim itself and is related to the repo

Acceptance Criteria

• bug fixed