fix initialization of ball_gaussians of rigidnodes

[MichTheEncoder]

Thanks for your wonderful job ! However, when I enabled the ball_gaussians option, an error was occured:

RuntimeError: Sizes of tensors must match except in dimension 0. Expected size 3 but got size 9 for tensor number 1 in the list.

After some troubleshooting, I found that the overwritten method, create_from_pcd, of the RigidNodes class incorrectly defined the dimensions of scale when initializing the scales of the ball gaussians. Therefore, I fix it as below: (I'm new to this field, so I can't make sure I'm right)