[Enhancement] Support loading GT for evaluation from multi-file backend (#867)

* support load gt for evaluation from multi-backend

* move some code from get_gt_seg_maps to get_one_gt_seg_map

* rename gt_seg_map_loader_conf to gt_seg_map_loader_cfg

* fix doc str

* rename get_one_gt_seg_map to get_gt_seg_map_by_idx