给出class-agnostic模块作用于faster_RCNN目标检测的pytorch训练示例代码，并给出注释

以下是class-agnostic模块作用于faster_RCNN目标检测的PyTorch训练示例代码，注释已经加入到代码中：

import torch
import torchvision
from torchvision.models.detection.faster_rcnn import FastRCNNPredictor
from torchvision.models.detection.mask_rcnn import MaskRCNNPredictor

# load a pre-trained model for classification and return only the features
backbone = torchvision.models.mobilenet_v2(pretrained=True).features

# FasterRCNN needs to know the number of output channels in a backbone.
# For mobilenet_v2, it's 1280, so we need to add it here
backbone.out_channels = 1280

# let's make the RPN generate 5 x 3 anchors per spatial location, with 5 different sizes and 3 different aspect ratios. 
# We have a Tuple[Tuple[int]] because each feature map could potentially have different sizes and aspect ratios 
# (e.g., if your backbone produces a few feature maps of different sizes).
anchor_generator = torchvision.models.detection.rpn.AnchorGenerator(sizes=((32, 64, 128, 256, 512),), 
                                                                    aspect_ratios=((0.5, 1.0, 2.0),))

# let's define what are the feature maps that we will use to perform the region of interest cropping, 
# as well as the size of the crop after rescaling.
# if your backbone returns a Tensor, featmap_names needs to be ['0']. More generally, the backbone should return an 
# OrderedDict[Tensor], and in featmap_names you can choose which feature maps to use.
roi_pooler = torchvision.ops.MultiScaleRoIAlign(featmap_names=['0'], 
                                                output_size=7, 
                                                sampling_ratio=2)

# put the pieces together inside a FasterRCNN model
model = torchvision.models.detection.FasterRCNN(backbone, 
                                                num_classes=2, 
                                                rpn_anchor_generator=anchor_generator, 
                                                box_roi_pool=roi_pooler)

# define a class-agnostic module
class ClassAgnosticModule(torch.nn.Module):
    def __init__(self, in_channels, num_classes):
        super().__init__()
        self.conv = torch.nn.Conv2d(in_channels, num_classes, kernel_size=1, stride=1, padding=0)
        
    def forward(self, x):
        # pass through the 1x1 convolution layer
        x = self.conv(x)
        # flatten the tensor
        x = x.flatten(start_dim=2)
        # apply softmax to get the class probabilities
        x = torch.nn.functional.softmax(x, dim=1)
        # reshape the tensor to match the output shape of the FasterRCNN model
        num_boxes = x.shape[1]
        x = x.reshape(-1, num_boxes, num_classes)
        return x

# replace the FastRCNNPredictor with the ClassAgnosticModule 
in_channels = model.roi_heads.box_predictor.cls_score.in_features
num_classes = 2
model.roi_heads.box_predictor = ClassAgnosticModule(in_channels, num_classes)

# define the loss function
def loss_fn(preds, targets):
    return torch.nn.functional.cross_entropy(preds.squeeze(), targets)

# define the optimizer
optimizer = torch.optim.Adam(model.parameters(), lr=0.001)

# define the data loader
data_loader = torch.utils.data.DataLoader(dataset, batch_size=4, shuffle=True)

# train the model
for epoch in range(num_epochs):
    for images, targets in data_loader:
        # move the images and targets to the device
        images = list(image.to(device) for image in images)
        targets = [{k: v.to(device) for k, v in t.items()} for t in targets]
        # forward pass
        preds = model(images, targets)
        # compute the loss
        loss_dict = preds['losses']
        losses = sum(loss_dict.values())
        # backward pass
        optimizer.zero_grad()
        losses.backward()
        optimizer.step()

该示例代码中，我们首先加载了一个预训练的分类模型，并删去了分类层。然后，我们定义了一个class-agnostic模块，并将FastRCNNPredictor替换为该模块。模型的其余部分与标准的FasterRCNN模型相同。最后，我们定义了一个损失函数和一个优化器，并使用数据加载器训练模型。

需要注意的是，该示例代码中的dataset和num_epochs变量没有给出，需要根据具体情况进行设置。