目标检测算法改进系列之Backbone替换为PoolFormer

PoolFormer

MetaFormer是颜水成大佬的一篇Transformer的论文，该篇论文的贡献主要有两点：第一、将Transformer抽象为一个通用架构的MetaFormer，并通过经验证明MetaFormer架构在Transformer/ mlp类模型取得了极大的成功。 第二、通过仅采用简单的非参数算子pooling作为MetaFormer的极弱token混合器，构建了一个名为PoolFormer。

原文地址：MetaFormer Is Actually What You Need for Vision

Transformer编码器如图1(a)所示，由两部分组成。一个是注意力模块，用于在token之间混合信息，我们将其称为token mixer。另一个组件包含剩余的模块，如通道mlp和残差连接。transformer的成功归功于基于注意力的token混合器。基于这一共识，已经开发了许多注意力模块的变体，以改进视觉Transformer，比如上篇DEiT就是增加了一个dist token。

最近的一些方法在MetaFormer架构中探索了其他类型的token mixers，例如，用傅里叶变换取代了注意力，仍然达到了普通transformer的约97%的精度。综合所有这些结果，似乎只要模型采用MetaFormer作为通用架构，就可以获得非常优秀的结果。为了验证这一假设，作者应用一个极其简单的非参数操作符pooling作为令牌混合器，只进行基本的令牌混合，将其命名为PoolFormer。PoolFormer-M36在ImageNet-1K分类基准上达到82.1%的top-1精度，超过了DeiT[53]和ResMLP[52]等调优的视觉变压器，充分展示了MetaFormer通用架构的优秀性能。

PoolFormer代码实现

# Copyright 2021 Garena Online Private Limited
#
# Licensed under the Apache License, Version 2.0 (the "License");
# you may not use this file except in compliance with the License.
# You may obtain a copy of the License at
#
#     http://www.apache.org/licenses/LICENSE-2.0
#
# Unless required by applicable law or agreed to in writing, software
# distributed under the License is distributed on an "AS IS" BASIS,
# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
# See the License for the specific language governing permissions and
# limitations under the License.
"""
PoolFormer implementation
"""
import os
import copy
import torch
import torch.nn as nn
import numpy as np

from timm.data import IMAGENET_DEFAULT_MEAN, IMAGENET_DEFAULT_STD
from timm.models.layers import DropPath, trunc_normal_, to_2tuple
from timm.models.registry import register_model

__all__ = ['poolformer_s12', 'poolformer_s24', 'poolformer_s36', 'poolformer_m48', 'poolformer_m36']

def _cfg(url='', **kwargs):
    return {
        'url': url,
        'num_classes': 1000, 'pool_size': None,
        'crop_pct': .95, 'interpolation': 'bicubic',
        'mean': IMAGENET_DEFAULT_MEAN, 'std': IMAGENET_DEFAULT_STD, 
        'classifier': 'head',
        **kwargs
    }


default_cfgs = {
    'poolformer_s': _cfg(crop_pct=0.9),
    'poolformer_m': _cfg(crop_pct=0.95),
}


class PatchEmbed(nn.Module):
    """
    Patch Embedding that is implemented by a layer of conv. 
    Input: tensor in shape [B, C, H, W]
    Output: tensor in shape [B, C, H/stride, W/stride]
    """
    def __init__(self, patch_size=16, stride=16, padding=0, 
                 in_chans=3, embed_dim=768, norm_layer=None):
        super().__init__()
        patch_size = to_2tuple(patch_size)
        stride = to_2tuple(stride)
        padding = to_2tuple(padding)
        self.proj = nn.Conv2d(in_chans, embed_dim, kernel_size=patch_size, 
                              stride=stride, padding=padding)
        self.norm = norm_layer(embed_dim) if norm_layer else nn.Identity()

    def forward(self, x):
        x = self.proj(x)
        x = self.norm(x)
        return x


class LayerNormChannel(nn.Module):
    """
    LayerNorm only for Channel Dimension.
    Input: tensor in shape [B, C, H, W]
    """
    def __init__(self, num_channels, eps=1e-05):
        super().__init__()
        self.weight = nn.Parameter(torch.ones(num_channels))
        self.bias = nn.Parameter(torch.zeros(num_channels))
        self.eps = eps

    def forward(self, x):
        u = x.mean(1, keepdim=True)
        s = (x - u).pow(2).mean(1, keepdim=True)
        x = (x - u) / torch.sqrt(s + self.eps)
        x = self.weight.unsqueeze(-1).unsqueeze(-1) * x \
            + self.bias.unsqueeze(-1).unsqueeze(-1)
        return x


class GroupNorm(nn.GroupNorm):
    """
    Group Normalization with 1 group.
    Input: tensor in shape [B, C, H, W]
    """
    def __init__(self, num_channels, **kwargs):
        super().__init__(1, num_channels, **kwargs)


class Pooling(nn.Module):
    """
    Implementation of pooling for PoolFormer
    --pool_size: pooling size
    """
    def __init__(self, pool_size=3):
        super().__init__()
        self.pool = nn.AvgPool2d(
            pool_size, stride=1, padding=pool_size//2, count_include_pad=False)

    def forward(self, x):
        return self.pool(x) - x


class Mlp(nn.Module):
    """
    Implementation of MLP with 1*1 convolutions.
    Input: tensor with shape [B, C, H, W]
    """
    def __init__(self, in_features, hidden_features=None, 
                 out_features=None, act_layer=nn.GELU, drop=0.):
        super().__init__()
        out_features = out_features or in_features
        hidden_features = hidden_features or in_features
        self.fc1 = nn.Conv2d(in_features, hidden_features, 1)
        self.act = act_layer()
        self.fc2 = nn.Conv2d(hidden_features, out_features, 1)
        self.drop = nn.Dropout(drop)
        self.apply(self._init_weights)

    def _init_weights(self, m):
        if isinstance(m, nn.Conv2d):
            trunc_normal_(m.weight, std=.02)
            if m.bias is not None:
                nn.init.constant_(m.bias, 0)

    def forward(self, x):
        x = self.fc1(x)
        x = self.act(x)
        x = self.drop(x)
        x = self.fc2(x)
        x = self.drop(x)
        return x


class PoolFormerBlock(nn.Module):
    """
    Implementation of one PoolFormer block.
    --dim: embedding dim
    --pool_size: pooling size
    --mlp_ratio: mlp expansion ratio
    --act_layer: activation
    --norm_layer: normalization
    --drop: dropout rate
    --drop path: Stochastic Depth, 
        refer to https://arxiv.org/abs/1603.09382
    --use_layer_scale, --layer_scale_init_value: LayerScale, 
        refer to https://arxiv.org/abs/2103.17239
    """
    def __init__(self, dim, pool_size=3, mlp_ratio=4., 
                 act_layer=nn.GELU, norm_layer=GroupNorm, 
                 drop=0., drop_path=0., 
                 use_layer_scale=True, layer_scale_init_value=1e-5):

        super().__init__()

        self.norm1 = norm_layer(dim)
        self.token_mixer = Pooling(pool_size=pool_size)
        self.norm2 = norm_layer(dim)
        mlp_hidden_dim = int(dim * mlp_ratio)
        self.mlp = Mlp(in_features=dim, hidden_features=mlp_hidden_dim, 
                       act_layer=act_layer, drop=drop)

        # The following two techniques are useful to train deep PoolFormers.
        self.drop_path = DropPath(drop_path) if drop_path > 0. \
            else nn.Identity()
        self.use_layer_scale = use_layer_scale
        if use_layer_scale:
            self.layer_scale_1 = nn.Parameter(
                layer_scale_init_value * torch.ones((dim)), requires_grad=True)
            self.layer_scale_2 = nn.Parameter(
                layer_scale_init_value * torch.ones((dim)), requires_grad=True)

    def forward(self, x):
        if self.use_layer_scale:
            x = x + self.drop_path(
                self.layer_scale_1.unsqueeze(-1).unsqueeze(-1)
                * self.token_mixer(self.norm1(x)))
            x = x + self.drop_path(
                self.layer_scale_2.unsqueeze(-1).unsqueeze(-1)
                * self.mlp(self.norm2(x)))
        else:
            x = x + self.drop_path(self.token_mixer(self.norm1(x)))
            x = x + self.drop_path(self.mlp(self.norm2(x)))
        return x


def basic_blocks(dim, index, layers, 
                 pool_size=3, mlp_ratio=4., 
                 act_layer=nn.GELU, norm_layer=GroupNorm, 
                 drop_rate=.0, drop_path_rate=0., 
                 use_layer_scale=True, layer_scale_init_value=1e-5):
    """
    generate PoolFormer blocks for a stage
    return: PoolFormer blocks 
    """
    blocks = []
    for block_idx in range(layers[index]):
        block_dpr = drop_path_rate * (
            block_idx + sum(layers[:index])) / (sum(layers) - 1)
        blocks.append(PoolFormerBlock(
            dim, pool_size=pool_size, mlp_ratio=mlp_ratio, 
            act_layer=act_layer, norm_layer=norm_layer, 
            drop=drop_rate, drop_path=block_dpr, 
            use_layer_scale=use_layer_scale, 
            layer_scale_init_value=layer_scale_init_value, 
            ))
    blocks = nn.Sequential(*blocks)

    return blocks


class PoolFormer(nn.Module):
    """
    PoolFormer, the main class of our model
    --layers: [x,x,x,x], number of blocks for the 4 stages
    --embed_dims, --mlp_ratios, --pool_size: the embedding dims, mlp ratios and 
        pooling size for the 4 stages
    --downsamples: flags to apply downsampling or not
    --norm_layer, --act_layer: define the types of normalization and activation
    --num_classes: number of classes for the image classification
    --in_patch_size, --in_stride, --in_pad: specify the patch embedding
        for the input image
    --down_patch_size --down_stride --down_pad: 
        specify the downsample (patch embed.)
    --fork_feat: whether output features of the 4 stages, for dense prediction
    --init_cfg, --pretrained: 
        for mmdetection and mmsegmentation to load pretrained weights
    """
    def __init__(self, layers, embed_dims=None, 
                 mlp_ratios=None, downsamples=None, 
                 pool_size=3, 
                 norm_layer=GroupNorm, act_layer=nn.GELU, 
                 num_classes=1000,
                 in_patch_size=7, in_stride=4, in_pad=2, 
                 down_patch_size=3, down_stride=2, down_pad=1, 
                 drop_rate=0., drop_path_rate=0.,
                 use_layer_scale=True, layer_scale_init_value=1e-5, 
                 fork_feat=True,
                 init_cfg=None, 
                 pretrained=None, 
                 **kwargs):

        super().__init__()

        if not fork_feat:
            self.num_classes = num_classes
        self.fork_feat = fork_feat

        self.patch_embed = PatchEmbed(
            patch_size=in_patch_size, stride=in_stride, padding=in_pad, 
            in_chans=3, embed_dim=embed_dims[0])

        # set the main block in network
        network = []
        for i in range(len(layers)):
            stage = basic_blocks(embed_dims[i], i, layers, 
                                 pool_size=pool_size, mlp_ratio=mlp_ratios[i],
                                 act_layer=act_layer, norm_layer=norm_layer, 
                                 drop_rate=drop_rate, 
                                 drop_path_rate=drop_path_rate,
                                 use_layer_scale=use_layer_scale, 
                                 layer_scale_init_value=layer_scale_init_value)
            network.append(stage)
            if i >= len(layers) - 1:
                break
            if downsamples[i] or embed_dims[i] != embed_dims[i+1]:
                # downsampling between two stages
                network.append(
                    PatchEmbed(
                        patch_size=down_patch_size, stride=down_stride, 
                        padding=down_pad, 
                        in_chans=embed_dims[i], embed_dim=embed_dims[i+1]
                        )
                    )

        self.network = nn.ModuleList(network)

        if self.fork_feat:
            # add a norm layer for each output
            self.out_indices = [0, 2, 4, 6]
            for i_emb, i_layer in enumerate(self.out_indices):
                if i_emb == 0 and os.environ.get('FORK_LAST3', None):
                    # TODO: more elegant way
                    """For RetinaNet, `start_level=1`. The first norm layer will not used.
                    cmd: `FORK_LAST3=1 python -m torch.distributed.launch ...`
                    """
                    layer = nn.Identity()
                else:
                    layer = norm_layer(embed_dims[i_emb])
                layer_name = f'norm{i_layer}'
                self.add_module(layer_name, layer)
        else:
            # Classifier head
            self.norm = norm_layer(embed_dims[-1])
            self.head = nn.Linear(
                embed_dims[-1], num_classes) if num_classes > 0 \
                else nn.Identity()
        self.init_cfg = copy.deepcopy(init_cfg)
        self.channel = [i.size(1) for i in self.forward(torch.randn(1, 3, 224, 224))]

    def reset_classifier(self, num_classes):
        self.num_classes = num_classes
        self.head = nn.Linear(
            self.embed_dim, num_classes) if num_classes > 0 else nn.Identity()

    def forward_embeddings(self, x):
        x = self.patch_embed(x)
        return x

    def forward_tokens(self, x):
        outs = []
        for idx, block in enumerate(self.network):
            x = block(x)
            if self.fork_feat and idx in self.out_indices:
                norm_layer = getattr(self, f'norm{idx}')
                x_out = norm_layer(x)
                outs.append(x_out)
        return outs

    def forward(self, x):
        # input embedding
        x = self.forward_embeddings(x)
        # through backbone
        x = self.forward_tokens(x)
        return x


model_urls = {
    "poolformer_s12": "https://github.com/sail-sg/poolformer/releases/download/v1.0/poolformer_s12.pth.tar",
    "poolformer_s24": "https://github.com/sail-sg/poolformer/releases/download/v1.0/poolformer_s24.pth.tar",
    "poolformer_s36": "https://github.com/sail-sg/poolformer/releases/download/v1.0/poolformer_s36.pth.tar",
    "poolformer_m36": "https://github.com/sail-sg/poolformer/releases/download/v1.0/poolformer_m36.pth.tar",
    "poolformer_m48": "https://github.com/sail-sg/poolformer/releases/download/v1.0/poolformer_m48.pth.tar",
}

def update_weight(model_dict, weight_dict):
    idx, temp_dict = 0, {}
    for k, v in weight_dict.items():
        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 poolformer_s12(pretrained=False, **kwargs):
    """
    PoolFormer-S12 model, Params: 12M
    --layers: [x,x,x,x], numbers of layers for the four stages
    --embed_dims, --mlp_ratios: 
        embedding dims and mlp ratios for the four stages
    --downsamples: flags to apply downsampling or not in four blocks
    """
    layers = [2, 2, 6, 2]
    embed_dims = [64, 128, 320, 512]
    mlp_ratios = [4, 4, 4, 4]
    downsamples = [True, True, True, True]
    model = PoolFormer(
        layers, embed_dims=embed_dims, 
        mlp_ratios=mlp_ratios, downsamples=downsamples, 
        **kwargs)
    model.default_cfg = default_cfgs['poolformer_s']
    if pretrained:
        url = model_urls['poolformer_s12']
        checkpoint = torch.hub.load_state_dict_from_url(url=url, map_location="cpu", check_hash=True)
        model.load_state_dict(update_weight(model.state_dict(), checkpoint))
    return model

def poolformer_s24(pretrained=False, **kwargs):
    """
    PoolFormer-S24 model, Params: 21M
    """
    layers = [4, 4, 12, 4]
    embed_dims = [64, 128, 320, 512]
    mlp_ratios = [4, 4, 4, 4]
    downsamples = [True, True, True, True]
    model = PoolFormer(
        layers, embed_dims=embed_dims, 
        mlp_ratios=mlp_ratios, downsamples=downsamples, 
        **kwargs)
    model.default_cfg = default_cfgs['poolformer_s']
    if pretrained:
        url = model_urls['poolformer_s24']
        checkpoint = torch.hub.load_state_dict_from_url(url=url, map_location="cpu", check_hash=True)
        model.load_state_dict(update_weight(model.state_dict(), checkpoint))
    return model

def poolformer_s36(pretrained=False, **kwargs):
    """
    PoolFormer-S36 model, Params: 31M
    """
    layers = [6, 6, 18, 6]
    embed_dims = [64, 128, 320, 512]
    mlp_ratios = [4, 4, 4, 4]
    downsamples = [True, True, True, True]
    model = PoolFormer(
        layers, embed_dims=embed_dims, 
        mlp_ratios=mlp_ratios, downsamples=downsamples, 
        layer_scale_init_value=1e-6, 
        **kwargs)
    model.default_cfg = default_cfgs['poolformer_s']
    if pretrained:
        url = model_urls['poolformer_s36']
        checkpoint = torch.hub.load_state_dict_from_url(url=url, map_location="cpu", check_hash=True)
        model.load_state_dict(update_weight(model.state_dict(), checkpoint))
    return model

def poolformer_m36(pretrained=False, **kwargs):
    """
    PoolFormer-M36 model, Params: 56M
    """
    layers = [6, 6, 18, 6]
    embed_dims = [96, 192, 384, 768]
    mlp_ratios = [4, 4, 4, 4]
    downsamples = [True, True, True, True]
    model = PoolFormer(
        layers, embed_dims=embed_dims, 
        mlp_ratios=mlp_ratios, downsamples=downsamples, 
        layer_scale_init_value=1e-6, 
        **kwargs)
    model.default_cfg = default_cfgs['poolformer_m']
    if pretrained:
        url = model_urls['poolformer_m36']
        checkpoint = torch.hub.load_state_dict_from_url(url=url, map_location="cpu", check_hash=True)
        model.load_state_dict(update_weight(model.state_dict(), checkpoint))
    return model


@register_model
def poolformer_m48(pretrained=False, **kwargs):
    """
    PoolFormer-M48 model, Params: 73M
    """
    layers = [8, 8, 24, 8]
    embed_dims = [96, 192, 384, 768]
    mlp_ratios = [4, 4, 4, 4]
    downsamples = [True, True, True, True]
    model = PoolFormer(
        layers, embed_dims=embed_dims, 
        mlp_ratios=mlp_ratios, downsamples=downsamples, 
        layer_scale_init_value=1e-6, 
        **kwargs)
    model.default_cfg = default_cfgs['poolformer_m']
    if pretrained:
        url = model_urls['poolformer_m48']
        checkpoint = torch.hub.load_state_dict_from_url(url=url, map_location="cpu", check_hash=True)
        model.load_state_dict(update_weight(model.state_dict(), checkpoint))
    return model

if __name__ == '__main__':
    model = poolformer_s12(pretrained=True)
    inputs = torch.randn((1, 3, 640, 640))
    for i in model(inputs):
        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 {poolformer_s12}: #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, poolformer_s12, [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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