feat(models): 纯卷积缺陷分类定位模型

面向实时设备部署，全程只用 convolution，不含 attention / transformer。

- backbones: 自研 tiny_conv（深度可分离，默认）与 torchvision 封装（resnet/mobilenet/shufflenet）
- necks: nearest 上采样 + 3x3 卷积，把特征恢复到 stride 4，兼顾小缺陷定位与速度
- heads: anchor-free 中心点定位头（heatmap + wh + offset）与图像级分类头；
  分类头输出维度随 label_mode 切换（多类多热 / 单类互斥含背景位）
- postprocess: 3x3 max-pool 峰值抑制替代 NMS 循环，无循环、可导出 TensorRT / OpenVINO

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

models/backbones/resnet.py

@@ -0,0 +1,73 @@
+"""torchvision 卷积骨架封装（resnet / mobilenet 等纯卷积模型）。"""
+
+from __future__ import annotations
+
+import torch
+from torch import nn
+
+from models.registry import BACKBONES
+
+# 仅允许纯卷积骨架；ViT/Swin 等不在部署范围内。
+_SUPPORTED = {
+    "resnet18": 512,
+    "resnet34": 512,
+    "resnet50": 2048,
+    "mobilenet_v3_small": 576,
+    "mobilenet_v3_large": 960,
+    "shufflenet_v2_x1_0": 1024,
+}
+
+
+@BACKBONES.register("torchvision")
+class TorchvisionBackbone(nn.Module):
+    """输出 stride=32 的单尺度特征。
+
+    Args:
+        arch: `_SUPPORTED` 中的模型名。
+        pretrained: 是否加载 ImageNet 权重（需要联网或本地缓存）。
+        in_channels: 输入通道，非 3 时会把第一层卷积权重按通道求均值复制。
+        frozen: 是否冻结骨架参数。
+    """
+
+    def __init__(
+        self,
+        arch: str = "resnet18",
+        pretrained: bool = True,
+        in_channels: int = 3,
+        frozen: bool = False,
+    ) -> None:
+        super().__init__()
+        if arch not in _SUPPORTED:
+            raise ValueError(f"不支持的骨架 {arch}，可选 {sorted(_SUPPORTED)}")
+
+        import torchvision
+
+        weights = "DEFAULT" if pretrained else None
+        net = getattr(torchvision.models, arch)(weights=weights)
+
+        if arch.startswith("resnet"):
+            self.features = nn.Sequential(*list(net.children())[:-2])
+        else:
+            self.features = net.features
+
+        if in_channels != 3:
+            self._patch_first_conv(in_channels)
+
+        self.out_channels = _SUPPORTED[arch]
+        self.out_stride = 32
+
+        if frozen:
+            for param in self.parameters():
+                param.requires_grad_(False)
+
+    def _patch_first_conv(self, in_channels: int) -> None:
+        for module in self.features.modules():
+            if isinstance(module, nn.Conv2d):
+                weight = module.weight.data.mean(dim=1, keepdim=True).repeat(1, in_channels, 1, 1)
+                module.in_channels = in_channels
+                module.weight = nn.Parameter(weight * (3.0 / in_channels))
+                return
+        raise RuntimeError("骨架中未找到 Conv2d，无法调整输入通道")
+
+    def forward(self, x: torch.Tensor) -> torch.Tensor:
+        return self.features(x)