[Proposal] Support Differential IK on the Newton backend #5451

Proposal

Add a PhysX-compatible get_jacobians() to isaaclab_newton's ArticulationView so DifferentialInverseKinematicsAction works on Newton, and drop the PhysX pin that's been used as a workaround in IK env configs (e.g. the Franka reach IK envs).

Motivation

DifferentialInverseKinematicsAction reads the Jacobian via self._asset.root_view.get_jacobians(). PhysX has it; Newton's ArticulationView (from newton.selection) only exposes eval_jacobian(), which returns the spatial Jacobian — so the existing call site doesn't work as-is on Newton, regardless of which robot is being used.

The current workaround is to pin affected envs to PhysX. The Franka reach IK envs do this explicitly:

# IK control is not supported with Newton physics; use PhysX only.
self.sim.physics = PhysxCfg(bounce_threshold_velocity=0.2)

It would be nice to remove that carve-out so IK envs run on either backend.

Alternatives

A clear and concise description of any alternative solutions or features you've considered, if any.

Build Info

Describe the versions where you are observing the missing feature in:

• Isaac Lab Version: 4.6.22
• isaaclab_newton Version: 0.5.25

Additional context

• Subclass newton.selection.ArticulationView in isaaclab_newton.assets.articulation and add get_jacobians().
• Inside it, call eval_jacobian(state), then convert spatial to geometric per body
• Return a wp.array so the existing wp.to_torch(...) path in task_space_actions.py keeps working unchanged.
• Re-export the adapter from isaaclab_newton.assets.articulation so Articulation picks it up instead of Newton's class directly.

Checklist

• I have checked that there is no similar issue in the repo (required)

Acceptance Criteria

(May be refined)

• isaaclab_newton's ArticulationView.get_jacobians() matches PhysX's shape and convention for fixed- and floating-base articulations.
• DifferentialInverseKinematicsAction runs on Newton without changes to its call site.
• The PhysX pin in the Franka reach IK envs can be removed.
• Test added comparing the Newton Jacobian against PhysX on a representative articulation.