Add body keyframe observation support in AMPLoader

[GiulioRomualdi]

Introduce support for body keyframe observations in AMPLoader, along with corresponding tests to ensure the functionality works as expected.

To use this feature, the user needs to specify the following dataset_cfg in the agent configuration:

dataset_cfg = {
    "amp_data_path": "/path/to/datasets",
    "datasets": {"walk_motion": 1.0},
    "slow_down_factor": 1,
    "amp_obs_components": ["joint_pos", "joint_vel", "base_lin_vel", "base_ang_vel", "body_pos_b", "body_ori_b"],
    "body_links_names": ["root_link", "left_knee", "right_knee", "left_foot", "right_foot"],
    "anchor_body_name": "root_link",
}

while, in the environment configuration:

@configclass
class AMPCfg(ObsGroup):
    joint_pos = ObsTerm(func=mdp.joint_pos)
    joint_vel = ObsTerm(func=mdp.joint_vel)
    base_lin_vel = ObsTerm(func=mdp.base_lin_vel)
    base_ang_vel = ObsTerm(func=mdp.base_ang_vel)
    body_pos_b = ObsTerm(func=gb_mdp.amp_body_pos_b, params={
        "anchor_body_name": "root_link",
        "body_names": ["root_link", "left_knee", "right_knee", "left_foot", "right_foot"],
    })
    body_ori_b = ObsTerm(func=gb_mdp.amp_body_ori_b, params={
        "anchor_body_name": "root_link",
        "body_names": ["root_link", "left_knee", "right_knee", "left_foot", "right_foot"],
    })

The user must ensure that amp_obs_components and the observation group produce the same observation vector. Moreover, the .npy prior should contain the required keyframe poses.

Notice that:

1. If specified, the keyframes are used to compute the relative transformation between the root link and each keyframe.
2. The orientation is provided to the discriminator network as a 6D vector, as suggested in On the Continuity of Rotation Representations in Neural Networks.

Important

The code is backward compatible: if amp_obs_components is not provided, the previous observation components will be used.

Tip

This feature can be used to enforce tracking of specific robot frames, such as end effectors.

[GiulioRomualdi]

This can be interesting for @gbionics/team-hermes and @gbionics/team-dexter

[LoreMoretti]

Looks good to me. Just a minor comment.

[LoreMoretti]

Isn't _compute_ang_vel more coherent here? In case, we should provide the full rotation matrix.