Ref: https://github.com/ydhongHIT/DDRNet#citation



Co-authored-by: xiexinch <xiexinch@outlook.com>

Thanks for your contribution and we appreciate it a lot. The following
instructions would make your pull request more healthy and more easily
get feedback. If you do not understand some items, don't worry, just
make the pull request and seek help from maintainers.

## Motivation

Support DDRNet
Paper: [Deep Dual-resolution Networks for Real-time and Accurate
Semantic Segmentation of Road Scenes](https://arxiv.org/pdf/2101.06085)
official Code: https://github.com/ydhongHIT/DDRNet


There is already a PR
https://github.com/open-mmlab/mmsegmentation/pull/1722 , but it has been
inactive for a long time.

## Current Result

### Cityscapes

#### inference with converted official weights

| Method | Backbone      | mIoU(official) | mIoU(converted weight) |
| ------ | ------------- | -------------- | ---------------------- |
| DDRNet | DDRNet23-slim | 77.8           | 77.84                  |
| DDRNet | DDRNet23 | 79.5 | 79.53 |

#### training with converted pretrained backbone

| Method | Backbone | Crop Size | Lr schd | Inf time(fps) | Device |
mIoU | mIoU(ms+flip) | config | download |
| ------ | ------------- | --------- | ------- | ------- | -------- |
----- | ------------- | ------------ | ------------ |
| DDRNet | DDRNet23-slim | 1024x1024 | 120000 | 85.85 | RTX 8000 | 77.85
| 79.80 |
[config](https://github.com/whu-pzhang/mmsegmentation/blob/ddrnet/configs/ddrnet/ddrnet_23-slim_in1k-pre_2xb6-120k_cityscapes-1024x1024.py)
| model \| log |
| DDRNet | DDRNet23 | 1024x1024 | 120000 | 33.41 | RTX 8000 | 79.53 |
80.98 |
[config](https://github.com/whu-pzhang/mmsegmentation/blob/ddrnet/configs/ddrnet/ddrnet_23_in1k-pre_2xb6-120k_cityscapes-1024x1024.py)
| model \| log |


The converted pretrained backbone weights download link:

1.
[ddrnet23s_in1k_mmseg.pth](https://drive.google.com/file/d/1Ni4F1PMGGjuld-1S9fzDTmneLfpMuPTG/view?usp=sharing)
2.
[ddrnet23_in1k_mmseg.pth](https://drive.google.com/file/d/11rsijC1xOWB6B0LgNQkAG-W6e1OdbCyJ/view?usp=sharing

)

## To do

- [x] support inference with converted official weights
- [x] support training on cityscapes dataset

---------

Co-authored-by: xiexinch <xiexinch@outlook.com>