Update link_poses type hint to Optional[Dict[str, Pose]]

[tylerlum]

• Updates link_poses type hint to be more correct. Before there was inconsistency (eg. List[pose], Dict[str, Pose], Dict[str, List[Pose]]), which can be confusing to new users because it wasn't 100% clear that this was the same thing as CudaRobotModelState.link_pose. From looking at the code, it is quite clear that these should all be Optional[Dict[str, Pose]]

[balakumar-s]

Made it Union[None, Dict[str,Pose]] to be more clear. While Optional can also be used, I find it confusing when it's the output of a function. We will push this in the next release.