Authors: Yongsa Chen
Description: DBNet_resnet18_fpn to be a base method.
1. Approximate the original polygons with coarser ones to speed up training.
2. Filter out some bad annotated samples in the training set.(training: validation is 9:1)
3. SGD and PolyLR Scheduler.
4. Randomly flip, rotate, ......
5. 20 training epochs.