loss很快降为零，但是map一直为零

训练自己的数据的时候loss很快就降到零了，map始终为零，求指教
![epoch_loss](https://github.com/bubbliiiing/detr-pytorch/assets/57293208/db551be7-5d7b-4546-9c88-d0c4b384c9f2)
![epoch_map](https://github.com/bubbliiiing/detr-pytorch/assets/57293208/1cc4c9c7-6872-49cc-9a81-7bf958a64399)
