The categories order between MSCOCO_format_annotation/500k_train.json and 500k_valid.json is different! #41

This lead to a very confused result during training and it's difficult to locate this hidden problem...Need modify the categories order to keep them same.
Though this is a small problem, but really influenced the experience of training. Hope @wolfshow @liminghao1630 can fix this and replace the .zip file