Fixes fetching actuator parameters from USD (#318)

# Description

This pull request fixes fetching actuator parameters from USD. Before,
when you had multiple actuator groups, each of which takes actuator
parameters directly from the articulation's USD, only the first actuator
group values were correctly set.

## Type of change

- Bug fix (non-breaking change which fixes an issue)

## Checklist

- [x] I have run the [`pre-commit` checks](https://pre-commit.com/) with
`./orbit.sh --format`
- [ ] I have made corresponding changes to the documentation
- [x] My changes generate no new warnings
- [ ] I have added tests that prove my fix is effective or that my
feature works
- [ ] I have updated the changelog and the corresponding version in the
extension's `config/extension.toml` file
- [x] I have added my name to the `CONTRIBUTORS.md` or my name already
exists there

source/extensions/omni.isaac.orbit/omni/isaac/orbit/assets/articulation/articulation.py

@@ -588,6 +588,13 @@ class Articulation(RigidObject):
         # flag for implicit actuators
         # if this is false, we by-pass certain checks when doing actuator-related operations
         self._has_implicit_actuators = False
+        # cache the values coming from the usd
+        usd_stiffness = self.root_physx_view.get_dof_stiffnesses().clone()
+        usd_damping = self.root_physx_view.get_dof_dampings().clone()
+        usd_armature = self.root_physx_view.get_dof_armatures().clone()
+        usd_friction = self.root_physx_view.get_dof_friction_coefficients().clone()
+        usd_effort_limit = self.root_physx_view.get_dof_max_forces().clone()
+        usd_velocity_limit = self.root_physx_view.get_dof_max_velocities().clone()
         # iterate over all actuator configurations
         for actuator_name, actuator_cfg in self.cfg.actuators.items():
             # type annotation for type checkers
@@ -608,12 +615,12 @@ class Articulation(RigidObject):
                 joint_ids=slice(None) if len(joint_names) == self.num_joints else joint_ids,
                 num_envs=self.num_instances,
                 device=self.device,
-                stiffness=self.root_physx_view.get_dof_stiffnesses()[:, joint_ids],
-                damping=self.root_physx_view.get_dof_dampings()[:, joint_ids],
-                armature=self.root_physx_view.get_dof_armatures()[:, joint_ids],
-                friction=self.root_physx_view.get_dof_friction_coefficients()[:, joint_ids],
-                effort_limit=self.root_physx_view.get_dof_max_forces()[:, joint_ids],
-                velocity_limit=self.root_physx_view.get_dof_max_velocities()[:, joint_ids],
+                stiffness=usd_stiffness[:, joint_ids],
+                damping=usd_damping[:, joint_ids],
+                armature=usd_armature[:, joint_ids],
+                friction=usd_friction[:, joint_ids],
+                effort_limit=usd_effort_limit[:, joint_ids],
+                velocity_limit=usd_velocity_limit[:, joint_ids],
             )
             # log information on actuator groups
             carb.log_info(