[FEATURE] expose joint position query

[Kashu7100]

Description

Add two new API get_joints_anchor_pos and get_joints_anchor_axis that returns the joint position in world coordinate and the axis direction in unit vector.

Related Issue

Resolves Genesis-Embodied-AI/Genesis#

Motivation and Context

How Has This Been / Can This Be Tested?

Screenshots (if appropriate):

Checklist:

• I read the CONTRIBUTING document.
• I followed the Submitting Code Changes section of CONTRIBUTING document.
• I tagged the title correctly (including BUG FIX/FEATURE/MISC/BREAKING)
• [] I updated the documentation accordingly or no change is needed.
• I tested my changes and added instructions on how to test it for reviewers.

• I have added tests to cover my changes.
• All new and existing tests passed.

[duburcqa]

Can you explain what is the use-case for this?

As already discussed several times in the past, I think exposing the “position” of joints is a bad idea because it is error prone. Most of the user will not understand what this is and will use it incorrectly. In my view, only link and geom poses should be exposed to the user.

[Kashu7100]

I was thinking of using this when I convert some code using different simulator but I think I can use the links to do the same!

[Kashu7100]

(Maybe) get_joints_anchor_pos is still worth keeping it. Feel free to reopen if needed.

[duburcqa]

Now that I remember, there is already RigidJoint.get_anchor_pos and RigidJoint.get_anchor_axis.

[duburcqa]

This means nothing: "This is the position of the joint in the world frame."

[duburcqa]

Here is the actual definition:

Genesis/genesis/engine/entities/rigid_entity/rigid_joint.py

Line 104 in 32904b3

Mathematically, the anchor point corresponds to the point that is fixed wrt parent link and is coincident with

[duburcqa]

After think twice, maybe we could factorize the implementations at joint-level with what you did? If you do so, I'm open to merge it!