fix sample code diff for OP(retinanet_detection_output, retinanet_target_assign), fix `gamma` default value for OP(sigmoid_focal_loss) !23744
Created by: FlyingQianMM
fix sample code diff for OP:
- retinanet_detection_output
- retinanet_target_assign
change gamma
default value from 2 to 2. for OP(sigmoid_focal_loss)
Chinese API pr: https://github.com/PaddlePaddle/FluidDoc/pull/1971
- retinanet_detection_output english api: