[Feature] Support DDRNet (open-mmlab#2855)

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## 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
open-mmlab#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>

configs/ddrnet/README.md

@@ -0,0 +1,47 @@
+# DDRNet
+
+> [Deep Dual-resolution Networks for Real-time and Accurate Semantic Segmentation of Road Scenes](http://arxiv.org/abs/2101.06085)
+
+## Introduction
+
+<!-- [ALGORITHM] -->
+
+<a href="https://github.com/ydhongHIT/DDRNet">Official Repo</a>
+
+## Abstract
+
+<!-- [ABSTRACT] -->
+
+Semantic segmentation is a key technology for autonomous vehicles to understand the surrounding scenes. The appealing performances of contemporary models usually come at the expense of heavy computations and lengthy inference time, which is intolerable for self-driving. Using light-weight architectures (encoder-decoder or two-pathway) or reasoning on low-resolution images, recent methods realize very fast scene parsing, even running at more than 100 FPS on a single 1080Ti GPU. However, there is still a significant gap in performance between these real-time methods and the models based on dilation backbones. To tackle this problem, we proposed a family of efficient backbones specially designed for real-time semantic segmentation. The proposed deep dual-resolution networks (DDRNets) are composed of two deep branches between which multiple bilateral fusions are performed. Additionally, we design a new contextual information extractor named Deep Aggregation Pyramid Pooling Module (DAPPM) to enlarge effective receptive fields and fuse multi-scale context based on low-resolution feature maps. Our method achieves a new state-of-the-art trade-off between accuracy and speed on both Cityscapes and CamVid dataset. In particular, on a single 2080Ti GPU, DDRNet-23-slim yields 77.4% mIoU at 102 FPS on Cityscapes test set and 74.7% mIoU at 230 FPS on CamVid test set. With widely used test augmentation, our method is superior to most state-of-the-art models and requires much less computation. Codes and trained models are available online.
+
+<!-- [IMAGE] -->
+
+<div align=center>
+<img src="https://raw.githubusercontent.com/ydhongHIT/DDRNet/main/figs/DDRNet_seg.png" width="80%"/>
+</div>
+
+## Results and models
+
+### Cityscapes
+
+| Method | Backbone      | Crop Size | Lr schd | Mem(GB) | Inf time(fps) | Device | mIoU  | mIoU(ms+flip) | config                                                                                                                                    | download                                                                                                                                                                                                                                                                                                                                                                                                             |
+| ------ | ------------- | --------- | ------- | ------- | ------------- | ------ | ----- | ------------- | ----------------------------------------------------------------------------------------------------------------------------------------- | -------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- |
+| DDRNet | DDRNet23-slim | 1024x1024 | 120000  | 1.70    | 85.85         | A100   | 77.84 | 80.15         | [config](https://github.com/open-mmlab/mmsegmentation/blob/main/configs/ddrnet/ddrnet_23-slim_in1k-pre_2xb6-120k_cityscapes-1024x1024.py) | [model](https://download.openmmlab.com/mmsegmentation/v0.5/ddrnet/ddrnet_23-slim_in1k-pre_2xb6-120k_cityscapes-1024x1024/ddrnet_23-slim_in1k-pre_2xb6-120k_cityscapes-1024x1024_20230426_145312-6a5e5174.pth) \| [log](https://download.openmmlab.com/mmsegmentation/v0.5/ddrnet/ddrnet_23-slim_in1k-pre_2xb6-120k_cityscapes-1024x1024/ddrnet_23-slim_in1k-pre_2xb6-120k_cityscapes-1024x1024_20230426_145312.json) |
+| DDRNet | DDRNet23      | 1024x1024 | 120000  | 7.26    | 33.41         | A100   | 79.99 | 81.71         | [config](https://github.com/open-mmlab/mmsegmentation/blob/main/configs/ddrnet/ddrnet_23_in1k-pre_2xb6-120k_cityscapes-1024x1024.py)      | [model](https://download.openmmlab.com/mmsegmentation/v0.5/ddrnet/ddrnet_23_in1k-pre_2xb6-120k_cityscapes-1024x1024/ddrnet_23_in1k-pre_2xb6-120k_cityscapes-1024x1024_20230425_162633-81601db0.pth) \| [log](https://download.openmmlab.com/mmsegmentation/v0.5/ddrnet/ddrnet_23_in1k-pre_2xb6-120k_cityscapes-1024x1024/ddrnet_23_in1k-pre_2xb6-120k_cityscapes-1024x1024_20230425_162633.json)                     |
+
+## Notes
+
+The pretrained weights in config files are converted from [the official repo](https://github.com/ydhongHIT/DDRNet#pretrained-models).
+
+## Citation
+
+```bibtex
+@misc{hong2021ddrnet,
+  title={Deep Dual-resolution Networks for Real-time and Accurate Semantic Segmentation of Road Scenes},
+  author={Hong, Yuanduo and Pan, Huihui and Sun, Weichao and Jia, Yisong},
+  year={2021},
+  eprint={2101.06085},
+  archivePrefix={arXiv},
+  primaryClass={cs.CV},
+}
+```

configs/ddrnet/ddrnet_23-slim_in1k-pre_2xb6-120k_cityscapes-1024x1024.py

@@ -0,0 +1,95 @@
+_base_ = [
+    '../_base_/datasets/cityscapes_1024x1024.py',
+    '../_base_/default_runtime.py',
+]
+
+# The class_weight is borrowed from https://github.com/openseg-group/OCNet.pytorch/issues/14 # noqa
+# Licensed under the MIT License
+class_weight = [
+    0.8373, 0.918, 0.866, 1.0345, 1.0166, 0.9969, 0.9754, 1.0489, 0.8786,
+    1.0023, 0.9539, 0.9843, 1.1116, 0.9037, 1.0865, 1.0955, 1.0865, 1.1529,
+    1.0507
+]
+
+crop_size = (1024, 1024)
+data_preprocessor = dict(
+    type='SegDataPreProcessor',
+    size=crop_size,
+    mean=[123.675, 116.28, 103.53],
+    std=[58.395, 57.12, 57.375],
+    bgr_to_rgb=True,
+    pad_val=0,
+    seg_pad_val=255)
+norm_cfg = dict(type='SyncBN', requires_grad=True)
+model = dict(
+    type='EncoderDecoder',
+    data_preprocessor=data_preprocessor,
+    backbone=dict(
+        type='DDRNet',
+        in_channels=3,
+        channels=32,
+        ppm_channels=128,
+        norm_cfg=norm_cfg,
+        align_corners=False,
+        init_cfg=dict(
+            type='Pretrained',
+            checkpoint='pretrained/ddrnet23s_in1k_mmseg.pth')),
+    decode_head=dict(
+        type='DDRHead',
+        in_channels=32 * 4,
+        channels=64,
+        dropout_ratio=0.,
+        num_classes=19,
+        align_corners=False,
+        norm_cfg=norm_cfg,
+        loss_decode=[
+            dict(
+                type='OhemCrossEntropy',
+                thres=0.9,
+                min_kept=131072,
+                class_weight=class_weight,
+                loss_weight=1.0),
+            dict(
+                type='OhemCrossEntropy',
+                thres=0.9,
+                min_kept=131072,
+                class_weight=class_weight,
+                loss_weight=0.4),
+        ]),
+
+    # model training and testing settings
+    train_cfg=dict(),
+    test_cfg=dict(mode='whole'))
+
+train_dataloader = dict(batch_size=6, num_workers=4)
+
+iters = 120000
+# optimizer
+optimizer = dict(type='SGD', lr=0.01, momentum=0.9, weight_decay=0.0005)
+optim_wrapper = dict(type='OptimWrapper', optimizer=optimizer, clip_grad=None)
+# learning policy
+param_scheduler = [
+    dict(
+        type='PolyLR',
+        eta_min=0,
+        power=0.9,
+        begin=0,
+        end=iters,
+        by_epoch=False)
+]
+
+# training schedule for 120k
+train_cfg = dict(
+    type='IterBasedTrainLoop', max_iters=iters, val_interval=iters // 10)
+val_cfg = dict(type='ValLoop')
+test_cfg = dict(type='TestLoop')
+default_hooks = dict(
+    timer=dict(type='IterTimerHook'),
+    logger=dict(type='LoggerHook', interval=50, log_metric_by_epoch=False),
+    param_scheduler=dict(type='ParamSchedulerHook'),
+    checkpoint=dict(
+        type='CheckpointHook', by_epoch=False, interval=iters // 10),
+    sampler_seed=dict(type='DistSamplerSeedHook'),
+    visualization=dict(type='SegVisualizationHook'))
+
+randomness = dict(seed=304)

configs/ddrnet/ddrnet_23_in1k-pre_2xb6-120k_cityscapes-1024x1024.py

@@ -0,0 +1,95 @@
+_base_ = [
+    '../_base_/datasets/cityscapes_1024x1024.py',
+    '../_base_/default_runtime.py',
+]
+
+# The class_weight is borrowed from https://github.com/openseg-group/OCNet.pytorch/issues/14 # noqa
+# Licensed under the MIT License
+class_weight = [
+    0.8373, 0.918, 0.866, 1.0345, 1.0166, 0.9969, 0.9754, 1.0489, 0.8786,
+    1.0023, 0.9539, 0.9843, 1.1116, 0.9037, 1.0865, 1.0955, 1.0865, 1.1529,
+    1.0507
+]
+
+crop_size = (1024, 1024)
+data_preprocessor = dict(
+    type='SegDataPreProcessor',
+    size=crop_size,
+    mean=[123.675, 116.28, 103.53],
+    std=[58.395, 57.12, 57.375],
+    bgr_to_rgb=True,
+    pad_val=0,
+    seg_pad_val=255)
+norm_cfg = dict(type='SyncBN', requires_grad=True)
+model = dict(
+    type='EncoderDecoder',
+    data_preprocessor=data_preprocessor,
+    backbone=dict(
+        type='DDRNet',
+        in_channels=3,
+        channels=64,
+        ppm_channels=128,
+        norm_cfg=norm_cfg,
+        align_corners=False,
+        init_cfg=dict(
+            type='Pretrained',
+            checkpoint='pretrained/ddrnet23_in1k_mmseg.pth')),
+    decode_head=dict(
+        type='DDRHead',
+        in_channels=64 * 4,
+        channels=128,
+        dropout_ratio=0.,
+        num_classes=19,
+        align_corners=False,
+        norm_cfg=norm_cfg,
+        loss_decode=[
+            dict(
+                type='OhemCrossEntropy',
+                thres=0.9,
+                min_kept=131072,
+                class_weight=class_weight,
+                loss_weight=1.0),
+            dict(
+                type='OhemCrossEntropy',
+                thres=0.9,
+                min_kept=131072,
+                class_weight=class_weight,
+                loss_weight=0.4),
+        ]),
+
+    # model training and testing settings
+    train_cfg=dict(),
+    test_cfg=dict(mode='whole'))
+
+train_dataloader = dict(batch_size=6, num_workers=4)
+
+iters = 120000
+# optimizer
+optimizer = dict(type='SGD', lr=0.01, momentum=0.9, weight_decay=0.0005)
+optim_wrapper = dict(type='OptimWrapper', optimizer=optimizer, clip_grad=None)
+# learning policy
+param_scheduler = [
+    dict(
+        type='PolyLR',
+        eta_min=0,
+        power=0.9,
+        begin=0,
+        end=iters,
+        by_epoch=False)
+]
+
+# training schedule for 120k
+train_cfg = dict(
+    type='IterBasedTrainLoop', max_iters=iters, val_interval=iters // 10)
+val_cfg = dict(type='ValLoop')
+test_cfg = dict(type='TestLoop')
+default_hooks = dict(
+    timer=dict(type='IterTimerHook'),
+    logger=dict(type='LoggerHook', interval=50, log_metric_by_epoch=False),
+    param_scheduler=dict(type='ParamSchedulerHook'),
+    checkpoint=dict(
+        type='CheckpointHook', by_epoch=False, interval=iters // 10),
+    sampler_seed=dict(type='DistSamplerSeedHook'),
+    visualization=dict(type='SegVisualizationHook'))
+
+randomness = dict(seed=304)

configs/ddrnet/metafile.yaml

@@ -0,0 +1,14 @@
+Collections:
+- Name: ''
+  License: Apache License 2.0
+  Metadata:
+    Training Data:
+    - Cityscapes
+  Paper:
+    Title: Deep Dual-resolution Networks for Real-time and Accurate Semantic Segmentation
+      of Road Scenes
+    URL: http://arxiv.org/abs/2101.06085
+  README: configs/ddrnet/README.md
+  Frameworks:
+  - PyTorch
+Models: []

mmseg/models/backbones/__init__.py

@@ -3,6 +3,7 @@
 from .bisenetv1 import BiSeNetV1
 from .bisenetv2 import BiSeNetV2
 from .cgnet import CGNet
+from .ddrnet import DDRNet
 from .erfnet import ERFNet
 from .fast_scnn import FastSCNN
 from .hrnet import HRNet
@@ -28,5 +29,6 @@
     'ResNeSt', 'MobileNetV2', 'UNet', 'CGNet', 'MobileNetV3',
     'VisionTransformer', 'SwinTransformer', 'MixVisionTransformer',
     'BiSeNetV1', 'BiSeNetV2', 'ICNet', 'TIMMBackbone', 'ERFNet', 'PCPVT',
-    'SVT', 'STDCNet', 'STDCContextPathNet', 'BEiT', 'MAE', 'PIDNet', 'MSCAN'
+    'SVT', 'STDCNet', 'STDCContextPathNet', 'BEiT', 'MAE', 'PIDNet', 'MSCAN',
+    'DDRNet'
 ]