Change some config for det trainning performance (#5055)

configs/deformable_detr/_base_/deformable_detr_reader.yml

-worker_num: 0
+worker_num: 2
 TrainReader:
   sample_transforms:
   - Decode: {}

configs/mot/jde/_base_/jde_reader_1088x608.yml

-worker_num: 2
+worker_num: 8
 TrainReader:
   sample_transforms:
     - Decode: {}

configs/solov2/_base_/solov2_reader.yml

-worker_num: 2
+worker_num: 8
 TrainReader:
   sample_transforms:
   - Decode: {}