Update picodet_lcnet_x2_5_640_mainbody.yml for release2.6 (#8301)

configs/picodet/legacy_model/application/mainbody_detection/picodet_lcnet_x2_5_640_mainbody.yml

 _BASE_: [
+  '../../../../datasets/coco_detection.yml',
   '../../../../runtime.yml',
   '../../_base_/picodet_esnet.yml',
   '../../_base_/optimizer_100e.yml',
@@ -20,23 +21,3 @@ PicoDet:
 LCNet:
   scale: 2.5
   feature_maps: [3, 4, 5]
-
-metric: COCO
-num_classes: 1
-
-TrainDataset:
-  !COCODataSet
-    image_dir: ./
-    anno_path: train.json
-    dataset_dir: dataset/mainbody/
-    data_fields: ['image', 'gt_bbox', 'gt_class', 'is_crowd']
-
-EvalDataset:
-  !COCODataSet
-    image_dir: ./
-    anno_path: val.json
-    dataset_dir: dataset/mainbody/
-
-TestDataset:
-  !ImageFolder
-    anno_path: ./dataset/mainbody/val.json