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# EfficientNetV2

## 1、drop_path方法

``````def drop_path(x, drop_prob: float = 0., training: bool = False):
if drop_prob == 0. or not training:
return x
keep_prob = 1 - drop_prob
shape = (x.shape[0],) + (1,) * (x.ndim - 1)  # work with diff dim tensors, not just 2D ConvNets
random_tensor = keep_prob + torch.rand(shape, dtype=x.dtype, device=x.device)
random_tensor.floor_()  # binarize
output = x.div(keep_prob) * random_tensor
return output
``````

## 2、DropPath类

``````class DropPath(nn.Module):
def __init__(self, drop_prob=None):
super(DropPath, self).__init__()
self.drop_prob = drop_prob

def forward(self, x):
return drop_path(x, self.drop_prob, self.training)
``````

## 3、ConvBNAct类

``````class ConvBNAct(nn.Module):
def __init__(self,
in_planes: int,
out_planes: int,
kernel_size: int = 3,
stride: int = 1,
groups: int = 1,
norm_layer: Optional[Callable[..., nn.Module]] = None,
activation_layer: Optional[Callable[..., nn.Module]] = None):
super(ConvBNAct, self).__init__()
padding = (kernel_size - 1) // 2
if norm_layer is None:
norm_layer = nn.BatchNorm2d
if activation_layer is None:
activation_layer = nn.SiLU  # alias Swish  (torch>=1.7)
self.conv = nn.Conv2d(in_channels=in_planes,
out_channels=out_planes,
kernel_size=kernel_size,
stride=stride,
groups=groups,
bias=False)
self.bn = norm_layer(out_planes)
self.act = activation_layer()
def forward(self, x):
result = self.conv(x)
result = self.bn(result)
result = self.act(result)
return result
``````

## 4、SE类

SE模块：首先，全局平均池化，将每一个channel求一个均值；再一次通过两个全连接层，得到输出。

``````class SqueezeExcite(nn.Module):
def __init__(self,
input_c: int,   # block input channel
expand_c: int,  # block expand channel
se_ratio: float = 0.25):
super(SqueezeExcite, self).__init__()
squeeze_c = int(input_c * se_ratio)
self.conv_reduce = nn.Conv2d(expand_c, squeeze_c, 1)
self.act1 = nn.SiLU()  # alias Swish
self.conv_expand = nn.Conv2d(squeeze_c, expand_c, 1)
self.act2 = nn.Sigmoid()

def forward(self, x: Tensor) -> Tensor:
scale = x.mean((2, 3), keepdim=True)
scale = self.conv_reduce(scale)
scale = self.act1(scale)
scale = self.conv_expand(scale)
scale = self.act2(scale)
return scale * x
``````

## 5、MBConv类

``````class MBConv(nn.Module):
def __init__(self,
kernel_size: int,
input_c: int,
out_c: int,
expand_ratio: int,
stride: int,
se_ratio: float,
drop_rate: float,
norm_layer: Callable[..., nn.Module]):
super(MBConv, self).__init__()
if stride not in [1, 2]:
raise ValueError("illegal stride value.")
self.has_shortcut = (stride == 1 and input_c == out_c)
activation_layer = nn.SiLU  # alias Swish
expanded_c = input_c * expand_ratio
# 在EfficientNetV2中，MBConv中不存在expansion=1的情况所以conv_pw肯定存在
assert expand_ratio != 1
# Point-wise expansion
self.expand_conv = ConvBNAct(input_c,
expanded_c,
kernel_size=1,
norm_layer=norm_layer,
activation_layer=activation_layer)
# Depth-wise convolution
self.dwconv = ConvBNAct(expanded_c,
expanded_c,
kernel_size=kernel_size,
stride=stride,
groups=expanded_c,
norm_layer=norm_layer,
activation_layer=activation_layer)
self.se = SqueezeExcite(input_c, expanded_c, se_ratio) if se_ratio > 0 else nn.Identity()
# Point-wise linear projection
self.project_conv = ConvBNAct(expanded_c,
out_planes=out_c,
kernel_size=1,
norm_layer=norm_layer,
activation_layer=nn.Identity)  # 注意这里没有激活函数，所有传入Identity
self.out_channels = out_c
# 只有在使用shortcut连接时才使用dropout层
self.drop_rate = drop_rate
if self.has_shortcut and drop_rate > 0:
self.dropout = DropPath(drop_rate)
def forward(self, x: Tensor) -> Tensor:
result = self.expand_conv(x)
result = self.dwconv(result)
result = self.se(result)
result = self.project_conv(result)
if self.has_shortcut:
if self.drop_rate > 0:
result = self.dropout(result)
result += x
return result
``````

## 6、FusedMBConv模块

``````class FusedMBConv(nn.Module):
def __init__(self,
kernel_size: int,
input_c: int,
out_c: int,
expand_ratio: int,
stride: int,
se_ratio: float,
drop_rate: float,
norm_layer: Callable[..., nn.Module]):
super(FusedMBConv, self).__init__()
assert stride in [1, 2]
assert se_ratio == 0
self.has_shortcut = stride == 1 and input_c == out_c
self.drop_rate = drop_rate
self.has_expansion = expand_ratio != 1
activation_layer = nn.SiLU  # alias Swish
expanded_c = input_c * expand_ratio
# 只有当expand ratio不等于1时才有expand conv
if self.has_expansion:
# Expansion convolution
self.expand_conv = ConvBNAct(input_c,
expanded_c,
kernel_size=kernel_size,
stride=stride,
norm_layer=norm_layer,
activation_layer=activation_layer)
self.project_conv = ConvBNAct(expanded_c,
out_c,
kernel_size=1,
norm_layer=norm_layer,
activation_layer=nn.Identity)  # 注意没有激活函数
else:
# 当只有project_conv时的情况
self.project_conv = ConvBNAct(input_c,
out_c,
kernel_size=kernel_size,
stride=stride,
norm_layer=norm_layer,
activation_layer=activation_layer)  # 注意有激活函数
self.out_channels = out_c
# 只有在使用shortcut连接时才使用dropout层
self.drop_rate = drop_rate
if self.has_shortcut and drop_rate > 0:
self.dropout = DropPath(drop_rate)
def forward(self, x: Tensor) -> Tensor:
if self.has_expansion:
result = self.expand_conv(x)
result = self.project_conv(result)
else:
result = self.project_conv(x)
if self.has_shortcut:
if self.drop_rate > 0:
result = self.dropout(result)
result += x
return result
``````

## 7、EfficientNetV2

``````class EfficientNetV2(nn.Module):
def __init__(self,
model_cnf: list,
num_classes: int = 1000,
num_features: int = 1280,
dropout_rate: float = 0.2,
drop_connect_rate: float = 0.2):
super(EfficientNetV2, self).__init__()

for cnf in model_cnf:
assert len(cnf) == 8

norm_layer = partial(nn.BatchNorm2d, eps=1e-3, momentum=0.1)

stem_filter_num = model_cnf[0][4]

self.stem = ConvBNAct(3,
stem_filter_num,
kernel_size=3,
stride=2,
norm_layer=norm_layer)  # 激活函数默认是SiLU

total_blocks = sum([i[0] for i in model_cnf])
block_id = 0
blocks = []
for cnf in model_cnf:
repeats = cnf[0]
op = FusedMBConv if cnf[-2] == 0 else MBConv
for i in range(repeats):
blocks.append(op(kernel_size=cnf[1],
input_c=cnf[4] if i == 0 else cnf[5],
out_c=cnf[5],
expand_ratio=cnf[3],
stride=cnf[2] if i == 0 else 1,
se_ratio=cnf[-1],
drop_rate=drop_connect_rate * block_id / total_blocks,
norm_layer=norm_layer))
block_id += 1
self.blocks = nn.Sequential(*blocks)

num_features,
kernel_size=1,
norm_layer=norm_layer)})  # 激活函数默认是SiLU

if dropout_rate > 0:

# initial weights
for m in self.modules():
if isinstance(m, nn.Conv2d):
nn.init.kaiming_normal_(m.weight, mode="fan_out")
if m.bias is not None:
nn.init.zeros_(m.bias)
elif isinstance(m, nn.BatchNorm2d):
nn.init.ones_(m.weight)
nn.init.zeros_(m.bias)
elif isinstance(m, nn.Linear):
nn.init.normal_(m.weight, 0, 0.01)
nn.init.zeros_(m.bias)

def forward(self, x: Tensor) -> Tensor:
x = self.stem(x)
x = self.blocks(x)

return x
``````