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import torch.nn as nn
import torch.nn.functional as F
def conv3x3(in_planes, out_planes, stride=1):
"""3x3 convolution with padding"""
return nn.Conv2d(in_planes, out_planes, kernel_size=3, stride=stride, padding=1, bias=False)
class BasicBlock(nn.Module):
def __init__(self, in_chan, out_chan, stride=1):
super(BasicBlock, self).__init__()
self.conv1 = conv3x3(in_chan, out_chan, stride)
self.bn1 = nn.BatchNorm2d(out_chan)
self.conv2 = conv3x3(out_chan, out_chan)
self.bn2 = nn.BatchNorm2d(out_chan)
self.relu = nn.ReLU(inplace=True)
self.downsample = None
if in_chan != out_chan or stride != 1:
self.downsample = nn.Sequential(
nn.Conv2d(in_chan, out_chan, kernel_size=1, stride=stride, bias=False),
nn.BatchNorm2d(out_chan),
)
def forward(self, x):
residual = self.conv1(x)
residual = F.relu(self.bn1(residual))
residual = self.conv2(residual)
residual = self.bn2(residual)
shortcut = x
if self.downsample is not None:
shortcut = self.downsample(x)
out = shortcut + residual
out = self.relu(out)
return out
def create_layer_basic(in_chan, out_chan, bnum, stride=1):
layers = [BasicBlock(in_chan, out_chan, stride=stride)]
for i in range(bnum - 1):
layers.append(BasicBlock(out_chan, out_chan, stride=1))
return nn.Sequential(*layers)
class ResNet18(nn.Module):
def __init__(self):
super(ResNet18, self).__init__()
self.conv1 = nn.Conv2d(3, 64, kernel_size=7, stride=2, padding=3, bias=False)
self.bn1 = nn.BatchNorm2d(64)
self.maxpool = nn.MaxPool2d(kernel_size=3, stride=2, padding=1)
self.layer1 = create_layer_basic(64, 64, bnum=2, stride=1)
self.layer2 = create_layer_basic(64, 128, bnum=2, stride=2)
self.layer3 = create_layer_basic(128, 256, bnum=2, stride=2)
self.layer4 = create_layer_basic(256, 512, bnum=2, stride=2)
def forward(self, x):
x = self.conv1(x)
x = F.relu(self.bn1(x))
x = self.maxpool(x)
x = self.layer1(x)
feat8 = self.layer2(x) # 1/8
feat16 = self.layer3(feat8) # 1/16
feat32 = self.layer4(feat16) # 1/32
return feat8, feat16, feat32
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