the formula of shape in _get_derivative of class BaseRankModel

[gmlyytt-YANG]

Hello Chenglong,

I red your code for several days. It's very good. But I was very confused when reading your code. Could you explain the definition of "shape" in function _get_derivative of class BaseRankModel, which locates at line 101-103 in ${root_dir}/src/model.py?

Thanks alot!