@Tony_Lee  for SfMPerspectiveCameras their wanted to ensure that  R_absolute  had valid rotation matrix, so they used an exponential map (implemented with  so3_exponential_map ) of the axis-angle representation of the rotation  log_R_absolute .
The boundle adjustment could be solved by recovered up to an unknown global rigid transformation g glob∈SE(3). Thus, for simplicity, they assumed knowledge of the absolute extrinsics of the first camera g0. They set g0 as a trivial camera g0=(I,0⃗ ) to work for simple rigid tranformation, but now that will change soon.
In order to be able to initialize the parameters you will need to use init for the class, .self and all the proper constructors. I am sure it will be fixed soon as they mentioned.
change to this class
class SfMPerspectiveCameras(CamerasBase): 
     def __init__(
        self,
        focal_length=1.0,
        principal_point=((0.0, 0.0),),
        R=_R,
        T=_T,
        device="cpu",
        image_size=((-1, -1),),
    ):

Hope it helps.