目标检测算法改进系列之Backbone替换为RepViT

RepViT简介

轻量级模型研究一直是计算机视觉任务中的一个焦点，其目标是在降低计算成本的同时达到优秀的性能。轻量级模型与资源受限的移动设备尤其相关，使得视觉模型的边缘部署成为可能。在过去十年中，研究人员主要关注轻量级卷积神经网络（CNNs）的设计，提出了许多高效的设计原则，包括可分离卷积 [2] 、逆瓶颈结构 [3] 、通道打乱 [4] 和结构重参数化 [5] 等，产生了 MobileNets [2, 3]，ShuffleNets [4] 和 RepVGG [5] 等代表性模型。

另一方面，视觉 Transformers（ViTs）成为学习视觉表征的另一种高效方案。与 CNNs 相比，ViTs 在各种计算机视觉任务中表现出了更优越的性能。然而，ViT 模型一般尺寸很大，延迟很高，不适合资源受限的移动设备。因此，研究人员开始探索 ViT 的轻量级设计。许多高效的ViTs设计原则被提出，大大提高了移动设备上 ViTs 的计算效率，产生了EfficientFormers [6] ，MobileViTs [7] 等代表性模型。这些轻量级 ViTs 在移动设备上展现出了相比 CNNs 的更强的性能和更低的延迟。

轻量级 ViTs 优于轻量级 CNNs 的原因通常归结于多头注意力模块，该模块使模型能够学习全局表征。然而，轻量级 ViTs 和轻量级 CNNs 在块结构、宏观和微观架构设计方面存在值得注意的差异，但这些差异尚未得到充分研究。这自然引出了一个问题：轻量级 ViTs 的架构选择能否提高轻量级 CNN 的性能？在这项工作中，我们结合轻量级 ViTs 的架构选择，重新审视了轻量级 CNNs 的设计。我们的旨在缩小轻量级 CNNs 与轻量级 ViTs 之间的差距，并强调前者与后者相比在移动设备上的应用潜力。

原文地址：RepViT: Revisiting Mobile CNN From ViT Perspective

RepViT代码实现

import torch.nn as nn
import numpy as np
from timm.models.layers import SqueezeExcite
import torch

__all__ = ['repvit_m1', 'repvit_m2', 'repvit_m3']

def replace_batchnorm(net):
    for child_name, child in net.named_children():
        if hasattr(child, 'fuse_self'):
            fused = child.fuse_self()
            setattr(net, child_name, fused)
            replace_batchnorm(fused)
        elif isinstance(child, torch.nn.BatchNorm2d):
            setattr(net, child_name, torch.nn.Identity())
        else:
            replace_batchnorm(child)

def _make_divisible(v, divisor, min_value=None):
    """
    This function is taken from the original tf repo.
    It ensures that all layers have a channel number that is divisible by 8
    It can be seen here:
    https://github.com/tensorflow/models/blob/master/research/slim/nets/mobilenet/mobilenet.py
    :param v:
    :param divisor:
    :param min_value:
    :return:
    """
    if min_value is None:
        min_value = divisor
    new_v = max(min_value, int(v + divisor / 2) // divisor * divisor)
    # Make sure that round down does not go down by more than 10%.
    if new_v < 0.9 * v:
        new_v += divisor
    return new_v

class Conv2d_BN(torch.nn.Sequential):
    def __init__(self, a, b, ks=1, stride=1, pad=0, dilation=1,
                 groups=1, bn_weight_init=1, resolution=-10000):
        super().__init__()
        self.add_module('c', torch.nn.Conv2d(
            a, b, ks, stride, pad, dilation, groups, bias=False))
        self.add_module('bn', torch.nn.BatchNorm2d(b))
        torch.nn.init.constant_(self.bn.weight, bn_weight_init)
        torch.nn.init.constant_(self.bn.bias, 0)

    @torch.no_grad()
    def fuse_self(self):
        c, bn = self._modules.values()
        w = bn.weight / (bn.running_var + bn.eps)**0.5
        w = c.weight * w[:, None, None, None]
        b = bn.bias - bn.running_mean * bn.weight / \
            (bn.running_var + bn.eps)**0.5
        m = torch.nn.Conv2d(w.size(1) * self.c.groups, w.size(
            0), w.shape[2:], stride=self.c.stride, padding=self.c.padding, dilation=self.c.dilation, groups=self.c.groups,
            device=c.weight.device)
        m.weight.data.copy_(w)
        m.bias.data.copy_(b)
        return m

class Residual(torch.nn.Module):
    def __init__(self, m, drop=0.):
        super().__init__()
        self.m = m
        self.drop = drop

    def forward(self, x):
        if self.training and self.drop > 0:
            return x + self.m(x) * torch.rand(x.size(0), 1, 1, 1,
                                              device=x.device).ge_(self.drop).div(1 - self.drop).detach()
        else:
            return x + self.m(x)
    
    @torch.no_grad()
    def fuse_self(self):
        if isinstance(self.m, Conv2d_BN):
            m = self.m.fuse_self()
            assert(m.groups == m.in_channels)
            identity = torch.ones(m.weight.shape[0], m.weight.shape[1], 1, 1)
            identity = torch.nn.functional.pad(identity, [1,1,1,1])
            m.weight += identity.to(m.weight.device)
            return m
        elif isinstance(self.m, torch.nn.Conv2d):
            m = self.m
            assert(m.groups != m.in_channels)
            identity = torch.ones(m.weight.shape[0], m.weight.shape[1], 1, 1)
            identity = torch.nn.functional.pad(identity, [1,1,1,1])
            m.weight += identity.to(m.weight.device)
            return m
        else:
            return self


class RepVGGDW(torch.nn.Module):
    def __init__(self, ed) -> None:
        super().__init__()
        self.conv = Conv2d_BN(ed, ed, 3, 1, 1, groups=ed)
        self.conv1 = Conv2d_BN(ed, ed, 1, 1, 0, groups=ed)
        self.dim = ed
    
    def forward(self, x):
        return self.conv(x) + self.conv1(x) + x
    
    @torch.no_grad()
    def fuse_self(self):
        conv = self.conv.fuse_self()
        conv1 = self.conv1.fuse_self()
        
        conv_w = conv.weight
        conv_b = conv.bias
        conv1_w = conv1.weight
        conv1_b = conv1.bias
        
        conv1_w = torch.nn.functional.pad(conv1_w, [1,1,1,1])

        identity = torch.nn.functional.pad(torch.ones(conv1_w.shape[0], conv1_w.shape[1], 1, 1, device=conv1_w.device), [1,1,1,1])

        final_conv_w = conv_w + conv1_w + identity
        final_conv_b = conv_b + conv1_b

        conv.weight.data.copy_(final_conv_w)
        conv.bias.data.copy_(final_conv_b)
        return conv


class RepViTBlock(nn.Module):
    def __init__(self, inp, hidden_dim, oup, kernel_size, stride, use_se, use_hs):
        super(RepViTBlock, self).__init__()
        assert stride in [1, 2]

        self.identity = stride == 1 and inp == oup
        assert(hidden_dim == 2 * inp)

        if stride == 2:
            self.token_mixer = nn.Sequential(
                Conv2d_BN(inp, inp, kernel_size, stride, (kernel_size - 1) // 2, groups=inp),
                SqueezeExcite(inp, 0.25) if use_se else nn.Identity(),
                Conv2d_BN(inp, oup, ks=1, stride=1, pad=0)
            )
            self.channel_mixer = Residual(nn.Sequential(
                    # pw
                    Conv2d_BN(oup, 2 * oup, 1, 1, 0),
                    nn.GELU() if use_hs else nn.GELU(),
                    # pw-linear
                    Conv2d_BN(2 * oup, oup, 1, 1, 0, bn_weight_init=0),
                ))
        else:
            assert(self.identity)
            self.token_mixer = nn.Sequential(
                RepVGGDW(inp),
                SqueezeExcite(inp, 0.25) if use_se else nn.Identity(),
            )
            self.channel_mixer = Residual(nn.Sequential(
                    # pw
                    Conv2d_BN(inp, hidden_dim, 1, 1, 0),
                    nn.GELU() if use_hs else nn.GELU(),
                    # pw-linear
                    Conv2d_BN(hidden_dim, oup, 1, 1, 0, bn_weight_init=0),
                ))

    def forward(self, x):
        return self.channel_mixer(self.token_mixer(x))

class RepViT(nn.Module):
    def __init__(self, cfgs):
        super(RepViT, self).__init__()
        # setting of inverted residual blocks
        self.cfgs = cfgs

        # building first layer
        input_channel = self.cfgs[0][2]
        patch_embed = torch.nn.Sequential(Conv2d_BN(3, input_channel // 2, 3, 2, 1), torch.nn.GELU(),
                           Conv2d_BN(input_channel // 2, input_channel, 3, 2, 1))
        layers = [patch_embed]
        # building inverted residual blocks
        block = RepViTBlock
        for k, t, c, use_se, use_hs, s in self.cfgs:
            output_channel = _make_divisible(c, 8)
            exp_size = _make_divisible(input_channel * t, 8)
            layers.append(block(input_channel, exp_size, output_channel, k, s, use_se, use_hs))
            input_channel = output_channel
        self.features = nn.ModuleList(layers)
        self.channel = [i.size(1) for i in self.forward(torch.randn(1, 3, 640, 640))]
        
    def forward(self, x):
        input_size = x.size(2)
        scale = [4, 8, 16, 32]
        features = [None, None, None, None]
        for f in self.features:
            x = f(x)
            if input_size // x.size(2) in scale:
                features[scale.index(input_size // x.size(2))] = x
        return features
    
    def switch_to_deploy(self):
        replace_batchnorm(self)

def update_weight(model_dict, weight_dict):
    idx, temp_dict = 0, {}
    for k, v in weight_dict.items():
        # k = k[9:]
        if k in model_dict.keys() and np.shape(model_dict[k]) == np.shape(v):
            temp_dict[k] = v
            idx += 1
    model_dict.update(temp_dict)
    print(f'loading weights... {idx}/{len(model_dict)} items')
    return model_dict

def repvit_m1(weights=''):
    """
    Constructs a MobileNetV3-Large model
    """
    cfgs = [
        # k, t, c, SE, HS, s 
        [3,   2,  48, 1, 0, 1],
        [3,   2,  48, 0, 0, 1],
        [3,   2,  48, 0, 0, 1],
        [3,   2,  96, 0, 0, 2],
        [3,   2,  96, 1, 0, 1],
        [3,   2,  96, 0, 0, 1],
        [3,   2,  96, 0, 0, 1],
        [3,   2,  192, 0, 1, 2],
        [3,   2,  192, 1, 1, 1],
        [3,   2,  192, 0, 1, 1],
        [3,   2,  192, 1, 1, 1],
        [3,   2, 192, 0, 1, 1],
        [3,   2, 192, 1, 1, 1],
        [3,   2, 192, 0, 1, 1],
        [3,   2, 192, 1, 1, 1],
        [3,   2, 192, 0, 1, 1],
        [3,   2, 192, 1, 1, 1],
        [3,   2, 192, 0, 1, 1],
        [3,   2, 192, 1, 1, 1],
        [3,   2, 192, 0, 1, 1],
        [3,   2, 192, 1, 1, 1],
        [3,   2, 192, 0, 1, 1],
        [3,   2, 192, 0, 1, 1],
        [3,   2, 384, 0, 1, 2],
        [3,   2, 384, 1, 1, 1],
        [3,   2, 384, 0, 1, 1]
    ]
    model = RepViT(cfgs)
    if weights:
        model.load_state_dict(update_weight(model.state_dict(), torch.load(weights)['model']))
    return model

def repvit_m2(weights=''):
    """
    Constructs a MobileNetV3-Large model
    """
    cfgs = [
        # k, t, c, SE, HS, s 
        [3,   2,  64, 1, 0, 1],
        [3,   2,  64, 0, 0, 1],
        [3,   2,  64, 0, 0, 1],
        [3,   2,  128, 0, 0, 2],
        [3,   2,  128, 1, 0, 1],
        [3,   2,  128, 0, 0, 1],
        [3,   2,  128, 0, 0, 1],
        [3,   2,  256, 0, 1, 2],
        [3,   2,  256, 1, 1, 1],
        [3,   2,  256, 0, 1, 1],
        [3,   2,  256, 1, 1, 1],
        [3,   2, 256, 0, 1, 1],
        [3,   2, 256, 1, 1, 1],
        [3,   2, 256, 0, 1, 1],
        [3,   2, 256, 1, 1, 1],
        [3,   2, 256, 0, 1, 1],
        [3,   2, 256, 1, 1, 1],
        [3,   2, 256, 0, 1, 1],
        [3,   2, 256, 1, 1, 1],
        [3,   2, 256, 0, 1, 1],
        [3,   2, 256, 0, 1, 1],
        [3,   2, 512, 0, 1, 2],
        [3,   2, 512, 1, 1, 1],
        [3,   2, 512, 0, 1, 1]
    ]
    model = RepViT(cfgs)
    if weights:
        model.load_state_dict(update_weight(model.state_dict(), torch.load(weights)['model']))
    return model

def repvit_m3(weights=''):
    """
    Constructs a MobileNetV3-Large model
    """
    cfgs = [
        # k, t, c, SE, HS, s 
        [3,   2,  64, 1, 0, 1],
        [3,   2,  64, 0, 0, 1],
        [3,   2,  64, 1, 0, 1],
        [3,   2,  64, 0, 0, 1],
        [3,   2,  64, 0, 0, 1],
        [3,   2,  128, 0, 0, 2],
        [3,   2,  128, 1, 0, 1],
        [3,   2,  128, 0, 0, 1],
        [3,   2,  128, 1, 0, 1],
        [3,   2,  128, 0, 0, 1],
        [3,   2,  128, 0, 0, 1],
        [3,   2,  256, 0, 1, 2],
        [3,   2,  256, 1, 1, 1],
        [3,   2,  256, 0, 1, 1],
        [3,   2,  256, 1, 1, 1],
        [3,   2,  256, 0, 1, 1],
        [3,   2,  256, 1, 1, 1],
        [3,   2,  256, 0, 1, 1],
        [3,   2,  256, 1, 1, 1],
        [3,   2, 256, 0, 1, 1],
        [3,   2, 256, 1, 1, 1],
        [3,   2, 256, 0, 1, 1],
        [3,   2, 256, 1, 1, 1],
        [3,   2, 256, 0, 1, 1],
        [3,   2, 256, 1, 1, 1],
        [3,   2, 256, 0, 1, 1],
        [3,   2, 256, 1, 1, 1],
        [3,   2, 256, 0, 1, 1],
        [3,   2, 256, 1, 1, 1],
        [3,   2, 256, 0, 1, 1],
        [3,   2, 256, 0, 1, 1],
        [3,   2, 512, 0, 1, 2],
        [3,   2, 512, 1, 1, 1],
        [3,   2, 512, 0, 1, 1]
    ]
    model = RepViT(cfgs)
    if weights:
        model.load_state_dict(update_weight(model.state_dict(), torch.load(weights)['model']))
    return model

if __name__ == '__main__':
    model = repvit_m1('repvit_m1_distill_300.pth')
    inputs = torch.randn((1, 3, 640, 640))
    res = model(inputs)
    for i in res:
        print(i.size())
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Backbone替换

yolo.py修改

def parse_model函数

def parse_model(d, ch):  # model_dict, input_channels(3)
    # Parse a YOLOv5 model.yaml dictionary
    LOGGER.info(f"\n{'':>3}{'from':>18}{'n':>3}{'params':>10}  {'module':<40}{'arguments':<30}")
    anchors, nc, gd, gw, act = d['anchors'], d['nc'], d['depth_multiple'], d['width_multiple'], d.get('activation')
    if act:
        Conv.default_act = eval(act)  # redefine default activation, i.e. Conv.default_act = nn.SiLU()
        LOGGER.info(f"{colorstr('activation:')} {act}")  # print
    na = (len(anchors[0]) // 2) if isinstance(anchors, list) else anchors  # number of anchors
    no = na * (nc + 5)  # number of outputs = anchors * (classes + 5)

    is_backbone = False
    layers, save, c2 = [], [], ch[-1]  # layers, savelist, ch out
    for i, (f, n, m, args) in enumerate(d['backbone'] + d['head']):  # from, number, module, args
        try:
            t = m
            m = eval(m) if isinstance(m, str) else m  # eval strings
        except:
            pass
        for j, a in enumerate(args):
            with contextlib.suppress(NameError):
                try:
                    args[j] = eval(a) if isinstance(a, str) else a  # eval strings
                except:
                    args[j] = a

        n = n_ = max(round(n * gd), 1) if n > 1 else n  # depth gain
        if m in {
                Conv, GhostConv, Bottleneck, GhostBottleneck, SPP, SPPF, DWConv, MixConv2d, Focus, CrossConv,
                BottleneckCSP, C3, C3TR, C3SPP, C3Ghost, nn.ConvTranspose2d, DWConvTranspose2d, C3x}:
            c1, c2 = ch[f], args[0]
            if c2 != no:  # if not output
                c2 = make_divisible(c2 * gw, 8)

            args = [c1, c2, *args[1:]]
            if m in {BottleneckCSP, C3, C3TR, C3Ghost, C3x}:
                args.insert(2, n)  # number of repeats
                n = 1
        elif m is nn.BatchNorm2d:
            args = [ch[f]]
        elif m is Concat:
            c2 = sum(ch[x] for x in f)
        # TODO: channel, gw, gd
        elif m in {Detect, Segment}:
            args.append([ch[x] for x in f])
            if isinstance(args[1], int):  # number of anchors
                args[1] = [list(range(args[1] * 2))] * len(f)
            if m is Segment:
                args[3] = make_divisible(args[3] * gw, 8)
        elif m is Contract:
            c2 = ch[f] * args[0] ** 2
        elif m is Expand:
            c2 = ch[f] // args[0] ** 2
        elif isinstance(m, str):
            t = m
            m = timm.create_model(m, pretrained=args[0], features_only=True)
            c2 = m.feature_info.channels()
        elif m in {repvit_m1}: #可添加更多Backbone
            m = m(*args)
            c2 = m.channel
        else:
            c2 = ch[f]
        if isinstance(c2, list):
            is_backbone = True
            m_ = m
            m_.backbone = True
        else:
            m_ = nn.Sequential(*(m(*args) for _ in range(n))) if n > 1 else m(*args)  # module
            t = str(m)[8:-2].replace('__main__.', '')  # module type
        np = sum(x.numel() for x in m_.parameters())  # number params
        m_.i, m_.f, m_.type, m_.np = i + 4 if is_backbone else i, f, t, np  # attach index, 'from' index, type, number params
        LOGGER.info(f'{i:>3}{str(f):>18}{n_:>3}{np:10.0f}  {t:<40}{str(args):<30}')  # print
        save.extend(x % (i + 4 if is_backbone else i) for x in ([f] if isinstance(f, int) else f) if x != -1)  # append to savelist
        layers.append(m_)
        if i == 0:
            ch = []
        if isinstance(c2, list):
            ch.extend(c2)
            for _ in range(5 - len(ch)):
                ch.insert(0, 0)
        else:
            ch.append(c2)
    return nn.Sequential(*layers), sorted(save)
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def _forward_once函数

def _forward_once(self, x, profile=False, visualize=False):
    y, dt = [], []  # outputs
    for m in self.model:
        if m.f != -1:  # if not from previous layer
            x = y[m.f] if isinstance(m.f, int) else [x if j == -1 else y[j] for j in m.f]  # from earlier layers
        if profile:
            self._profile_one_layer(m, x, dt)
        if hasattr(m, 'backbone'):
            x = m(x)
            for _ in range(5 - len(x)):
                x.insert(0, None)
            for i_idx, i in enumerate(x):
                if i_idx in self.save:
                    y.append(i)
                else:
                    y.append(None)
            x = x[-1]
        else:
            x = m(x)  # run
            y.append(x if m.i in self.save else None)  # save output
        if visualize:
            feature_visualization(x, m.type, m.i, save_dir=visualize)
    return x
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创建.yaml配置文件

# YOLOv5 🚀 by Ultralytics, GPL-3.0 license

# Parameters
nc: 80  # number of classes
depth_multiple: 0.33  # model depth multiple
width_multiple: 0.25  # layer channel multiple
anchors:
  - [10,13, 16,30, 33,23]  # P3/8
  - [30,61, 62,45, 59,119]  # P4/16
  - [116,90, 156,198, 373,326]  # P5/32

# 0-P1/2
# 1-P2/4
# 2-P3/8
# 3-P4/16
# 4-P5/32

# YOLOv5 v6.0 backbone
backbone:
  # [from, number, module, args]
  [[-1, 1, repvit_m1, [False]], # 4
   [-1, 1, SPPF, [1024, 5]],  # 5
  ]

# YOLOv5 v6.0 head
head:
  [[-1, 1, Conv, [512, 1, 1]], # 6
   [-1, 1, nn.Upsample, [None, 2, 'nearest']], # 7
   [[-1, 3], 1, Concat, [1]],  # cat backbone P4 8
   [-1, 3, C3, [512, False]],  # 9

   [-1, 1, Conv, [256, 1, 1]], # 10
   [-1, 1, nn.Upsample, [None, 2, 'nearest']], # 11
   [[-1, 2], 1, Concat, [1]],  # cat backbone P3 12
   [-1, 3, C3, [256, False]],  # 13 (P3/8-small)

   [-1, 1, Conv, [256, 3, 2]], # 14
   [[-1, 10], 1, Concat, [1]],  # cat head P4 15
   [-1, 3, C3, [512, False]],  # 16 (P4/16-medium)

   [-1, 1, Conv, [512, 3, 2]], # 17
   [[-1, 5], 1, Concat, [1]],  # cat head P5 18
   [-1, 3, C3, [1024, False]],  # 19 (P5/32-large)

   [[13, 16, 19], 1, Detect, [nc, anchors]],  # Detect(P3, P4, P5)
  ]
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