Clarifies asset classes' default_inertia tensor coordinate frame (#3405)

# Description

The default_inertia attributes of the Articulation,
RigidObjectCollection, and RigidObject data asset classes did not
specify in what coordinate frame the tensors should be provided. This PR
addresses this, and addresses some minor inconsistencies across the
default_inertia docstrings.

## Type of change

- This change requires a documentation update

## Screenshots

ArticulationData

| Before | After |
| ------ | ----- |
| <img width="731" height="162" alt="image"
src="https://github.com/user-attachments/assets/7e2a6973-6264-4ba0-89d7-fd903e3bee6d"
/>| <img width="750" height="262" alt="image"
src="https://github.com/user-attachments/assets/470832e6-0164-4402-b4c7-6a5ac5e599a1"
/>|

RigidObjectCollectionData

| Before | After |
| ------ | ----- |
| <img width="731" height="181" alt="image"
src="https://github.com/user-attachments/assets/7d6bd039-b7f5-40e2-9180-33d0748694a8"
/> | <img width="733" height="276" alt="image"
src="https://github.com/user-attachments/assets/07bb32ca-628c-4132-8009-1db1c279d653"
/> |

RigidObjectData

| Before | After |
| ------ | ----- |
| <img width="732" height="160" alt="image"
src="https://github.com/user-attachments/assets/43b29f1c-3e92-4b96-925f-88cdf7ef3fc3"
/> | <img width="733" height="276" alt="image"
src="https://github.com/user-attachments/assets/d16bbc57-3939-4f2e-84cb-9ec135dcbe7e"
/> |


## Checklist

- [x] I have run the [`pre-commit` checks](https://pre-commit.com/) with
`./isaaclab.sh --format`
- [x] 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

CONTRIBUTORS.md

@@ -111,6 +111,7 @@ Guidelines for modifications:
 * Özhan Özen
 * Patrick Yin
 * Peter Du
+* Philipp Reist
 * Pulkit Goyal
 * Qian Wan
 * Qinxi Yu

source/isaaclab/isaaclab/assets/articulation/articulation_data.py

@@ -151,8 +151,9 @@ class ArticulationData:
     default_inertia: torch.Tensor = None
     """Default inertia for all the bodies in the articulation. Shape is (num_instances, num_bodies, 9).
 
-    The inertia is the inertia tensor relative to the center of mass frame. The values are stored in
-    the order :math:`[I_{xx}, I_{xy}, I_{xz}, I_{yx}, I_{yy}, I_{yz}, I_{zx}, I_{zy}, I_{zz}]`.
+    The inertia tensor should be given with respect to the center of mass, expressed in the articulation links' actor frame.
+    The values are stored in the order :math:`[I_{xx}, I_{yx}, I_{zx}, I_{xy}, I_{yy}, I_{zy}, I_{xz}, I_{yz}, I_{zz}]`.
+    However, due to the symmetry of inertia tensors, row- and column-major orders are equivalent.
 
     This quantity is parsed from the USD schema at the time of initialization.
     """

source/isaaclab/isaaclab/assets/rigid_object/rigid_object_data.py

@@ -112,8 +112,11 @@ class RigidObjectData:
     default_inertia: torch.Tensor = None
     """Default inertia tensor read from the simulation. Shape is (num_instances, 9).
 
-    The inertia is the inertia tensor relative to the center of mass frame. The values are stored in
-    the order :math:`[I_{xx}, I_{xy}, I_{xz}, I_{yx}, I_{yy}, I_{yz}, I_{zx}, I_{zy}, I_{zz}]`.
+    The inertia tensor should be given with respect to the center of mass, expressed in the rigid body's actor frame.
+    The values are stored in the order :math:`[I_{xx}, I_{yx}, I_{zx}, I_{xy}, I_{yy}, I_{zy}, I_{xz}, I_{yz}, I_{zz}]`.
+    However, due to the symmetry of inertia tensors, row- and column-major orders are equivalent.
+
+    This quantity is parsed from the USD schema at the time of initialization.
     """
 
     ##

source/isaaclab/isaaclab/assets/rigid_object_collection/rigid_object_collection_data.py

@@ -118,8 +118,11 @@ class RigidObjectCollectionData:
     default_inertia: torch.Tensor = None
     """Default object inertia tensor read from the simulation. Shape is (num_instances, num_objects, 9).
 
-    The inertia is the inertia tensor relative to the center of mass frame. The values are stored in
-    the order :math:`[I_{xx}, I_{xy}, I_{xz}, I_{yx}, I_{yy}, I_{yz}, I_{zx}, I_{zy}, I_{zz}]`.
+    The inertia tensor should be given with respect to the center of mass, expressed in the rigid body's actor frame.
+    The values are stored in the order :math:`[I_{xx}, I_{yx}, I_{zx}, I_{xy}, I_{yy}, I_{zy}, I_{xz}, I_{yz}, I_{zz}]`.
+    However, due to the symmetry of inertia tensors, row- and column-major orders are equivalent.
+
+    This quantity is parsed from the USD schema at the time of initialization.
     """
 
     ##