Fix ONNX demo postprocessing

[artur-ag]

Same fix as here from @shiboyang

Tested with

python deploy/export_onnx.py \
  configs/pretrain/yolo_world_v2_l_vlpan_bn_2e-3_100e_4x8gpus_obj365v1_goldg_train_lvis_minival.py \
  demo/pretrained_weights/yolo_world_v2_l_obj365v1_goldg_pretrain-a82b1fe3.pth \
  --custom-text custom-text.json \
  --opset 12 --without-nms

python deploy/onnx_demo.py \
  work_dirs/yolo_world_v2_l_obj365v1_goldg_pretrain-a82b1fe3.onnx \
  <INPUT> custom-text.json \
  --onnx-nms