Fix projected gravity calculation in legged_robot.py

[curieuxjy]

(Update legged_robot.py)

The misfunction for calculating projected gravity should be resolved by changing quat_rotate_inverse to the quat_rotate function for calculating the projected_gravity vector.

[SUZ-tsinghua]

No. It is quat_rotate_inverse.

[curieuxjy]

No. It is quat_rotate_inverse.

@SUZ-tsinghua, I wanted to bring your attention to a specific line of code in the NVIDIA-Omniverse IsaacGymEnvs repository. Have you had a chance to review it?

Here is the link for reference: https://github.com/NVIDIA-Omniverse/IsaacGymEnvs/blob/aeed298638a1f7b5421b38f5f3cc2d1079b6d9c3/isaacgymenvs/tasks/anymal.py#L372.

In this updated version of the repository, you'll notice that the function quat_rotate_inverse has been changed to quat_rotate. Additionally, if you implement a visualization for the projected gravity vector and observe the rendering window, the error becomes evident. This observation suggests that the initial implementation using quat_rotate_inverse was wrong.

[SUZ-tsinghua]

No. It is quat_rotate_inverse.

@SUZ-tsinghua, I wanted to bring your attention to a specific line of code in the NVIDIA-Omniverse IsaacGymEnvs repository. Have you had a chance to review it?

Here is the link for reference: https://github.com/NVIDIA-Omniverse/IsaacGymEnvs/blob/aeed298638a1f7b5421b38f5f3cc2d1079b6d9c3/isaacgymenvs/tasks/anymal.py#L372.

In this updated version of the repository, you'll notice that the function quat_rotate_inverse has been changed to ``quat_rotate. Additionally, if you implement a visualization for the projected gravity vector and observe the rendering window, the error becomes evident. This observation suggests that the initial implementation using quat_rotate_inverse` was wrong.

@curieuxjy But you can directly verify the correctness of the code by running
`from isaacgym.torch_utils import *

base_quat = torch.tensor([[0.0, 0.7071, 0.0, 0.7071]])
gravity_vec = torch.tensor([[0.0, 0.0, -1.]])

projected_gravity_inverse = quat_rotate_inverse(base_quat, gravity_vec)
projected_gravity = quat_rotate(base_quat, gravity_vec)

print("projected gravity inverse",projected_gravity_inverse)
print("projected gravity", projected_gravity)``

It rotates the base frame so the x axis of the base frame is the -z axis of the env frame. And projected_gravity_inverse is [1,0,0] but projected_gravity is [-1,0,0].