Drop the unreachable Newton preset from the Digit tasks

[AntoineRichard]

Problem

LocomotionVelocityRoughEnvCfg declares events.base_com as a preset whose
newton_mjwarp branch disables the centre-of-mass randomization, because Newton does
not support it:

base_com = preset(
    default=EventTerm(func=mdp.randomize_rigid_body_com, ...),
    newton_mjwarp=None,
)

On tasks that offer Newton, that branch is a companion: it rides along with the
newton_mjwarp already on the physics axis and is never selectable on its own.

Digit is PhysX-only — DigitPhysicsCfg declares only isaacsim_physx, physx and
default, and its own docstring says so. The inherited branch therefore names no
reachable backend:

token	result
physics=newton_mjwarp	rejected — the task has no Newton physics variant
presets=newton_mjwarp	resolves, stays on PhysxCfg, and silently drops the randomization

So the three Digit tasks advertised a presets=newton_mjwarp that was never a backend
switch. Selecting it gave you PhysX physics with a Newton-only workaround applied.

Affects IsaacContrib-Velocity-Flat-Digit, IsaacContrib-Velocity-Rough-Digit and
IsaacContrib-Tracking-LocoManip-Digit — all three derive from DigitRoughEnvCfg.

Fix

Collapse the preset to its default in DigitRoughEnvCfg.__post_init__, so the task
offers only what it can run. One edit covers all three tasks.

Verification

• All three tasks now report presets: [] (was ['newton_mjwarp']); base_com
  survives with body_names="torso_base".

• AnymalCRoughEnvCfg still carries ['default', 'newton_mjwarp'] — the shared
  config and every task that does offer Newton are untouched.

• New test_digit_physics_presets.py follows the existing
  test_dr_legs_physics_presets.py pattern (offline, no sim). Confirmed the guard
  fails without the fix and passes with it:

  1 failed, 2 passed   # fix reverted
  3 passed             # fix applied
  

Notes

Found while building a task-discovery API that enumerates each task's runnable backend
combinations — this surfaced as a task declaring a preset it cannot resolve.

Sibling velocity tasks were checked: Velocity-Flat-AnymalC has the same base_com
preset and newton_mjwarp on its physics axis, so the companion is correctly paired
there; G1 and Spot override base_com away entirely. Digit was the only one that
inherited the companion without the backend.

[isaaclab-review-bot]

Isaac Lab Review Bot

Reviewed the Digit-specific collapse of the inherited base_com preset and its changelog entry. The change narrowly removes the standalone Newton workaround from the three PhysX-only Digit tasks while preserving their default center-of-mass randomization.

• Design and architecture: Applying the override in DigitRoughEnvCfg.__post_init__ keeps the shared locomotion configuration and Newton-capable tasks unchanged. This is appropriately scoped to the Digit configuration.
• API: The three Digit tasks will no longer expose or accept presets=newton_mjwarp. Existing scripts that explicitly pass that token will fail rather than silently disabling randomization on PhysX; the changelog clearly documents this compatibility change.
• Implementation: After the superclass initialization, events.base_com is rebound from the preset wrapper to its default EventTerm, then its asset body is configured as torso_base. This preserves the effective default PhysX event while removing the unreachable preset branch.

No blocking issues. No inline issue met the actionable-evidence threshold; the assessment above records the review feedback.

Automated review; human maintainers own approval decisions.

[greptile-apps]

Greptile Summary

The PR removes an unreachable Newton-specific preset from PhysX-only Digit tasks while retaining center-of-mass randomization with the Digit torso body configuration.

• Collapses the inherited base_com preset to its default event term in DigitRoughEnvCfg.
• Documents that the affected Digit velocity and loco-manipulation tasks no longer advertise the standalone newton_mjwarp preset.

Confidence Score: 5/5

The PR appears safe to merge with no actionable defects identified.

The preset collapse occurs on an instance-isolated configuration, removes the unwanted preset from discovery, preserves the default event and its Digit-specific body selection, and does not violate assumptions in current Digit subclasses.

Important Files Changed

Filename	Overview
source/isaaclab_tasks/isaaclab_tasks/contrib/velocity/config/digit/rough_env_cfg.py	Safely replaces the unreachable preset wrapper with its default event term and preserves the existing torso_base targeting.
source/isaaclab_tasks/changelog.d/fix-digit-newton-companion.rst	Accurately documents the affected tasks, prior incorrect preset behavior, and scope of the fix.

_{Reviews (1): Last reviewed commit: "Address review: shorten the Digit preset..." | Re-trigger Greptile}