Change to use F-string (#2531)

mmdet/apis/inference.py

mmdet/apis/test.py

mmdet/apis/train.py

mmdet/core/anchor/anchor_generator.py

mmdet/core/bbox/assigners/assign_result.py

mmdet/core/bbox/iou_calculators/iou2d_calculator.py

mmdet/core/bbox/samplers/sampling_result.py

mmdet/core/evaluation/class_names.py

mmdet/core/evaluation/eval_hooks.py

mmdet/core/evaluation/mean_ap.py

mmdet/core/evaluation/recall.py

mmdet/core/mask/structures.py

mmdet/core/optimizer/default_constructor.py

mmdet/datasets/cityscapes.py

mmdet/datasets/coco.py

mmdet/datasets/custom.py

mmdet/datasets/pipelines/compose.py

mmdet/datasets/pipelines/formating.py

mmdet/datasets/pipelines/instaboost.py

mmdet/datasets/pipelines/loading.py