DETR源码解读

transformer由encoder和decoder俩部分组成。

Encoder

一个encoder由多个encoder_layer组成，在detr中默认是6层。

class TransformerEncoder(nn.Module):

def __init__(self, encoder_layer, num_layers, norm=None):
        super().__init__()
        self.layers = _get_clones(encoder_layer, num_layers) # Encoder包含num层，每层具有相同结构encoder_layer
        self.num_layers = num_layers

self.norm = norm # 归一化

def forward(self, src,
                mask: Optional[Tensor] = None,
                src_key_padding_mask: Optional[Tensor] = None,
                pos: Optional[Tensor] = None):

# src 对应backbone最后一层输出的feature maps,并且维度已经映射到(h*w,bs, hidden_dim)
        # mask 一般为空
        # pos 对应backbone最后一层输出的feature maps对应的位置编码，shape是(h*w,bs,c)
        # src_key_padding_mask 对应backbone最后一层输出的feature maps对应的mask，shape是(bs,h*w)

output = src

for layer in self.layers:
            output = layer(output, src_mask=mask,
                           src_key_padding_mask=src_key_padding_mask, pos=pos)

if self.norm is not None:
            output = self.norm(output)

return output

EncoderLayer 的前向过程分为两种情况，一种是在输入多头自注意力层和前向反馈层前先进行归一化，另一种则是在这两个层输出后再进行归一化操作。对应实现可以参考如下图左侧部分：

class TransformerEncoderLayer(nn.Module):

def __init__(self, d_model, nhead, dim_feedforward=2048, dropout=0.1,
                 activation="relu", normalize_before=False):
        super().__init__()

#多头自注意力模块
        self.self_attn = nn.MultiheadAttention(d_model, nhead, dropout=dropout)
        # Implementation of Feedforward model
        self.linear1 = nn.Linear(d_model, dim_feedforward)
        self.dropout = nn.Dropout(dropout)
        self.linear2 = nn.Linear(dim_feedforward, d_model)

self.norm1 = nn.LayerNorm(d_model)
        self.norm2 = nn.LayerNorm(d_model)
        self.dropout1 = nn.Dropout(dropout)
        self.dropout2 = nn.Dropout(dropout)

self.activation = _get_activation_fn(activation)

# 是否需要在输入多头自注意力层之前进行归一化
        self.normalize_before = normalize_before

def with_pos_embed(self, tensor, pos: Optional[Tensor]):
        return tensor if pos is None else tensor + pos

def forward_post(self,
                     src,
                     src_mask: Optional[Tensor] = None,
                     src_key_padding_mask: Optional[Tensor] = None,
                     pos: Optional[Tensor] = None):
        q = k = self.with_pos_embed(src, pos)
        src2 = self.self_attn(q, k, value=src, attn_mask=src_mask,
                              key_padding_mask=src_key_padding_mask)[0]
        src = src + self.dropout1(src2)
        src = self.norm1(src)
        src2 = self.linear2(self.dropout(self.activation(self.linear1(src))))
        src = src + self.dropout2(src2)
        src = self.norm2(src)
        return src

def forward_pre(self, src,
                    src_mask: Optional[Tensor] = None,
                    src_key_padding_mask: Optional[Tensor] = None,
                    pos: Optional[Tensor] = None):

# 输入多头自注意力层前进行归一化
        src2 = self.norm1(src)
        # q,k在输入attn之前需要结合位置编码
        q = k = self.with_pos_embed(src2, pos)
        # self.self_attn是nn.MultiheadAttention的实例，其前向过程返回两部分，第一个是自注意力层的输出，第二个是自注意力权重，因此这里取了输出索引为0的部分即代表自注意力层的输出。
        src2 = self.self_attn(q, k, value=src2, attn_mask=src_mask,
                              key_padding_mask=src_key_padding_mask)[0]

src = src + self.dropout1(src2)
        src2 = self.norm2(src)
        src2 = self.linear2(self.dropout(self.activation(self.linear1(src2))))
        src = src + self.dropout2(src2)
        return src

def forward(self, src,
                src_mask: Optional[Tensor] = None,
                src_key_padding_mask: Optional[Tensor] = None,
                pos: Optional[Tensor] = None):

# 俩种不同的前向过程
        if self.normalize_before:
            return self.forward_pre(src, src_mask, src_key_padding_mask, pos)
        return self.forward_post(src, src_mask, src_key_padding_mask, pos)

需要注意的是，在输入多头自注意力层时需要先进行位置嵌入，即结合位置编码。注意仅对query和key实施，而value不需要。query和key是在图像特征中各个位置之间计算相关性，而value作为原图像特征，使用计算出来的相关性加权上去，得到各位置结合了全局相关性（增强/削弱）后的特征表示。

Query Embedding

在解析Decoder前，有必要先简要地谈谈query embedding，因为它是Decoder的主要输入之一。query embedding 有点anchor的味道，而且是自学习的anchor，作者使用了nn.Embedding实现：

self.query_embed = nn.Embedding(num_queries, hidden_dim)

其中num_queries 代表图像中有多少个目标（位置），默认是100个，对这些目标（位置）全部进行嵌入，维度映射到 hidden_dim，将 query_embedding 的权重作为参数输入到Transformer的前向过程，使用时与position encoding的方式相同：直接相加。

hs = self.transformer(self.input_proj(src), mask, self.query_embed.weight, pos[-1])[0]

而这个query embedding应该加在哪呢？当然是我们需要预测的目标（query object）咯！可是网络一开始还没有输出，我们都不知道预测目标在哪里呀，如何将它实体化？作者也不知道，于是就简单粗暴地直接将它初始化为全0，shape和query embedding 的权重一致（从而可以element-wise add）。

class Transformer(nn.Module):

def __init__(self, d_model=512, nhead=8, num_encoder_layers=6,
                 num_decoder_layers=6, dim_feedforward=2048, dropout=0.1,
                 activation="relu", normalize_before=False,
                 return_intermediate_dec=False):
        super().__init__()
    ...
    def forward(self, src, mask, query_embed, pos_embed):

...
        # (num_queries,bs,hidden_dim)
        tgt = torch.zeros_like(query_embed) #
        memory = self.encoder(src, src_key_padding_mask=mask, pos=pos_embed)
        hs = self.decoder(tgt, memory, memory_key_padding_mask=mask,
                          pos=pos_embed, query_pos=query_embed)
        return hs.transpose(1, 2), memory.permute(1, 2, 0).view(bs, c, h, w)

Decoder

Decoder的结构和Encoder十分类似。

class TransformerDecoder(nn.Module):

def __init__(self, decoder_layer, num_layers, norm=None, return_intermediate=False):
        super().__init__()
        self.layers = _get_clones(decoder_layer, num_layers)
        self.num_layers = num_layers
        self.norm = norm
        self.return_intermediate = return_intermediate

def forward(self, tgt, memory,
                tgt_mask: Optional[Tensor] = None,
                memory_mask: Optional[Tensor] = None,
                tgt_key_padding_mask: Optional[Tensor] = None,
                memory_key_padding_mask: Optional[Tensor] = None,
                pos: Optional[Tensor] = None,
                query_pos: Optional[Tensor] = None):

# tgt 是query embedding，shape是(num_queries,bs,hidden_dim)
        # query_pos 是对应tgt的位置编码，shape和tgt一致
        # memory是encoder的输出，shape是(h*w,bs,hidden_dim)
        # memory_key_padding_mask是对应encoder的src_key_padding_mask，shape是(bs,h*w)
        # pos 对应输入到encoder的位置编码，这里代表memory的位置编码，shape和memory一致

output = tgt

intermediate = []

for layer in self.layers:
            output = layer(output, memory, tgt_mask=tgt_mask,
                           memory_mask=memory_mask,
                           tgt_key_padding_mask=tgt_key_padding_mask,
                           memory_key_padding_mask=memory_key_padding_mask,
                           pos=pos, query_pos=query_pos)
            if self.return_intermediate:
                intermediate.append(self.norm(output))

if self.norm is not None:
            output = self.norm(output)
            if self.return_intermediate:
                intermediate.pop()
                intermediate.append(output)

if self.return_intermediate:
            return torch.stack(intermediate)
        return output.unsqueeze(0)

注意，在detr中，tgt_mask和memory_mask并未使用。需要注意的是intermediate中记录的是每层输出后的归一化结果，而每一层的输入是前一层输出（没有归一化）的结果。

DecoderLayer与Encoder的实现类似，只不过多了一层cross attention，其实质也是多头自注意力层，但是key和value来自于Encoder的输出。

class TransformerDecoderLayer(nn.Module):

def __init__(self, d_model, nhead, dim_feedforward=2048, dropout=0.1,
                 activation="relu", normalize_before=False):
        super().__init__()
        self.self_attn = nn.MultiheadAttention(d_model, nhead, dropout=dropout)
        self.multihead_attn = nn.MultiheadAttention(d_model, nhead, dropout=dropout)
        # Implementation of Feedforward model
        self.linear1 = nn.Linear(d_model, dim_feedforward)
        self.dropout = nn.Dropout(dropout)
        self.linear2 = nn.Linear(dim_feedforward, d_model)

self.norm1 = nn.LayerNorm(d_model)
        self.norm2 = nn.LayerNorm(d_model)
        self.norm3 = nn.LayerNorm(d_model)
        self.dropout1 = nn.Dropout(dropout)
        self.dropout2 = nn.Dropout(dropout)
        self.dropout3 = nn.Dropout(dropout)

self.activation = _get_activation_fn(activation)
        self.normalize_before = normalize_before

def with_pos_embed(self, tensor, pos: Optional[Tensor]):
        return tensor if pos is None else tensor + pos

def forward_post(self, tgt, memory,
                     tgt_mask: Optional[Tensor] = None,
                     memory_mask: Optional[Tensor] = None,
                     tgt_key_padding_mask: Optional[Tensor] = None,
                     memory_key_padding_mask: Optional[Tensor] = None,
                     pos: Optional[Tensor] = None,
                     query_pos: Optional[Tensor] = None):
        q = k = self.with_pos_embed(tgt, query_pos)
        tgt2 = self.self_attn(q, k, value=tgt, attn_mask=tgt_mask,
                              key_padding_mask=tgt_key_padding_mask)[0]
        tgt = tgt + self.dropout1(tgt2)
        tgt = self.norm1(tgt)
        tgt2 = self.multihead_attn(query=self.with_pos_embed(tgt, query_pos),
                                   key=self.with_pos_embed(memory, pos),
                                   value=memory, attn_mask=memory_mask,
                                   key_padding_mask=memory_key_padding_mask)[0]
        tgt = tgt + self.dropout2(tgt2)
        tgt = self.norm2(tgt)
        tgt2 = self.linear2(self.dropout(self.activation(self.linear1(tgt))))
        tgt = tgt + self.dropout3(tgt2)
        tgt = self.norm3(tgt)
        return tgt

def forward_pre(self, tgt, memory,
                    tgt_mask: Optional[Tensor] = None,
                    memory_mask: Optional[Tensor] = None,
                    tgt_key_padding_mask: Optional[Tensor] = None,
                    memory_key_padding_mask: Optional[Tensor] = None,
                    pos: Optional[Tensor] = None,
                    query_pos: Optional[Tensor] = None):

tgt2 = self.norm1(tgt)
        # 进行位置嵌入
        q = k = self.with_pos_embed(tgt2, query_pos)
        # 多头自注意力层，输入不包含encoder的输出
        tgt2 = self.self_attn(q, k, value=tgt2, attn_mask=tgt_mask,
                              key_padding_mask=tgt_key_padding_mask)[0]
        tgt = tgt + self.dropout1(tgt2)
        tgt2 = self.norm2(tgt)
        # cross attention，key,value来自encoder，query来自上一层输出
        # key,query均需进行位置嵌入
        tgt2 = self.multihead_attn(query=self.with_pos_embed(tgt2, query_pos),
                                   key=self.with_pos_embed(memory, pos),
                                   value=memory, attn_mask=memory_mask,
                                   key_padding_mask=memory_key_padding_mask)[0]
        tgt = tgt + self.dropout2(tgt2)
        tgt2 = self.norm3(tgt)
        tgt2 = self.linear2(self.dropout(self.activation(self.linear1(tgt2))))
        tgt = tgt + self.dropout3(tgt2)
        return tgt

def forward(self, tgt, memory,
                tgt_mask: Optional[Tensor] = None,
                memory_mask: Optional[Tensor] = None,
                tgt_key_padding_mask: Optional[Tensor] = None,
                memory_key_padding_mask: Optional[Tensor] = None,
                pos: Optional[Tensor] = None,
                query_pos: Optional[Tensor] = None):
        if self.normalize_before:
            return self.forward_pre(tgt, memory, tgt_mask, memory_mask,
                                    tgt_key_padding_mask, memory_key_padding_mask, pos, query_pos)
        return self.forward_post(tgt, memory, tgt_mask, memory_mask,
                                 tgt_key_padding_mask, memory_key_padding_mask, pos, query_pos)

注意，在tgt在输入到self_attn之前，需要经过position embedding，tgt+query_pos。在第二个多头注意力模块multihead_attn上，key和value均来自Encoder的输出。同样地，query和key要进行位置嵌入（而value不用）。这里cross attention计算的相关性是目标物体与图像特征各位置的相关性，然后再把这个相关性系数加权到Encoder编码后的图像特征（value）上，相当于获得了object features的意思，更好地表征了图像中的各个物体。从上面encoder和decoder的实现可以看出，作者非常强调位置嵌入的作用，每次进行attention计算前都需要进行position embedding，究其原因是因为transformer的转置不变性，即对排列和位置是不care的，然而在detection任务中却是十分重要的。

Transformer

将Encoder和Decoder封装在一起构成Transformer。

class Transformer(nn.Module):

def __init__(self, d_model=512, nhead=8, num_encoder_layers=6,
                 num_decoder_layers=6, dim_feedforward=2048, dropout=0.1,
                 activation="relu", normalize_before=False,
                 return_intermediate_dec=False):
        super().__init__()
		# 构建encoder layer
        encoder_layer = TransformerEncoderLayer(d_model, nhead, dim_feedforward,
                                                dropout, activation, normalize_before)
        encoder_norm = nn.LayerNorm(d_model) if normalize_before else None
        self.encoder = TransformerEncoder(encoder_layer, num_encoder_layers, encoder_norm)
		#构建decoder layer
        decoder_layer = TransformerDecoderLayer(d_model, nhead, dim_feedforward,
                                                dropout, activation, normalize_before)
        decoder_norm = nn.LayerNorm(d_model)
        self.decoder = TransformerDecoder(decoder_layer, num_decoder_layers, decoder_norm,
                                          return_intermediate=return_intermediate_dec)

self._reset_parameters()

self.d_model = d_model # 输入的embedding的特征维度
        self.nhead = nhead #

def _reset_parameters(self):
        for p in self.parameters():
            if p.dim() > 1:
                nn.init.xavier_uniform_(p)

def forward(self, src, mask, query_embed, pos_embed):
        # flatten NxCxHxW to HWxNxC
        bs, c, h, w = src.shape
        # 将backbone输入的feature maps进行flatten成序列，
        # src: (h*w,bs,c)
        src = src.flatten(2).permute(2, 0, 1)
        # pos: (h*w,bs,hidden_dim)
        pos_embed = pos_embed.flatten(2).permute(2, 0, 1)
        # query_embed: (num_queries, bs, hidden_dim)
        query_embed = query_embed.unsqueeze(1).repeat(1, bs, 1)

# mask: (bs, h*w)
        mask = mask.flatten(1)

tgt = torch.zeros_like(query_embed) # 每次forward时，tgt都会初始化为0
        # memory: (h*w, bs, c)
        memory = self.encoder(src, src_key_padding_mask=mask, pos=pos_embed)
        # TransformerDecoderLayer中return_intermediate设置为true，因此decoder包含了每层的输出结果，因此hs的shape是（6, num_queries,bs,hidden_dim）
        hs = self.decoder(tgt, memory, memory_key_padding_mask=mask,
                          pos=pos_embed, query_pos=query_embed)
        return hs.transpose(1, 2), memory.permute(1, 2, 0).view(bs, c, h, w)

注意，tgt是与query embedding形状一直且设置为全0的结果，意为初始化需要预测的目标。因为一开始并不清楚这些目标，所以初始化为全0。其会在Decoder的各层不断被refine，相当于一个coarse-to-fine的过程，但是真正要学习的是query embedding，学习到的是整个数据集中目标物体的统计特征，而tgt在每次迭代训练（一个batch数据刚到来）时会被重新初始化为0。

DETR包含backbone，encoder, decoder, prediction heads四个部分。encoder和decoder通常会用一个transformer来实现。prediction heads部分包括分类和回归。

class DETR(nn.Module):
    """ This is the DETR module that performs object detection """
    def __init__(self, backbone, transformer, num_classes, num_queries, aux_loss=False):
        """ Initializes the model.
        Parameters:
            backbone: torch module of the backbone to be used. See backbone.py
            transformer: torch module of the transformer architecture. See transformer.py
            num_classes: number of object classes
            num_queries: number of object queries, ie detection slot. This is the maximal number of objects
                         DETR can detect in a single image. For COCO, we recommend 100 queries.
            aux_loss: True if auxiliary decoding losses (loss at each decoder layer) are to be used.
        """
        super().__init__()
        self.num_queries = num_queries
        self.transformer = transformer
        hidden_dim = transformer.d_model
        # class分类
        self.class_embed = nn.Linear(hidden_dim, num_classes + 1)
        # box回归，包含3层nn.linear()，最后一层维度映射为4，代表bbox的中心点横、纵坐标和宽、高。
        self.bbox_embed = MLP(hidden_dim, hidden_dim, 4, 3)
        # query_embed用于在Transformer中对初始化query以及对其编码生成嵌入
        self.query_embed = nn.Embedding(num_queries, hidden_dim)
		# input_proj是将CNN提取的特征维度映射到Transformer隐层的维度；
        self.input_proj = nn.Conv2d(backbone.num_channels, hidden_dim, kernel_size=1)
        self.backbone = backbone
        self.aux_loss = aux_loss

def forward(self, samples: NestedTensor):
        # 将sample转换成nestedTensor类型
        if isinstance(samples, (list, torch.Tensor)):
            samples = nested_tensor_from_tensor_list(samples)
        # 输入cnn提取特征，并输出pos encoding  
        features, pos = self.backbone(samples)
		# 取出最后一层特征及对应mask
        src, mask = features[-1].decompose()
        assert mask is not None
        hs = self.transformer(self.input_proj(src), mask, self.query_embed.weight, pos[-1])[0]

# 生成分类与回归的预测结果
        outputs_class = self.class_embed(hs)
        outputs_coord = self.bbox_embed(hs).sigmoid()
        # 由于hs包含transformer中decoder每层输出，因此索引-1表示取最后一层输出
        out = {'pred_logits': outputs_class[-1], 'pred_boxes': outputs_coord[-1]}

if self.aux_loss:
            out['aux_outputs'] = self._set_aux_loss(outputs_class, outputs_coord)
        return out

Postprocess

一部分DETR的输出并不是最终预测结果的形式，还需要进行简单的后处理。但是这里的后处理并不是NMS哦！DETR预测的是集合，并且在训练过程中经过匈牙利算法与GT一对一匹配学习，因此不存在重复框的情况。

class PostProcess(nn.Module):
    """ This module converts the model's output into the format expected by the coco api"""
    @torch.no_grad()
    def forward(self, outputs, target_sizes):

out_logits, out_bbox = outputs['pred_logits'], outputs['pred_boxes']
        assert len(out_logits) == len(target_sizes)
        assert target_sizes.shape[1] == 2
		# out_logits : (bs, num_queries,num_classes)
        prob = F.softmax(out_logits, -1)
        scores, labels = prob[..., :-1].max(-1)

# convert to [x0, y0, x1, y1] format
        boxes = box_ops.box_cxcywh_to_xyxy(out_bbox)
        # and from relative [0, 1] to absolute [0, height] coordinates
        img_h, img_w = target_sizes.unbind(1)
        scale_fct = torch.stack([img_w, img_h, img_w, img_h], dim=1)
        boxes = boxes * scale_fct[:, None, :]

results = [{'scores': s, 'labels': l, 'boxes': b} for s, l, b in zip(scores, labels, boxes)]

return results

Loss Fuction

这一部分主要介绍一下和损失函数相关的部分源码。先看一下与损失函数相关的代码：

matcher = build_matcher(args)
weight_dict = {'loss_ce': 1, 'loss_bbox': args.bbox_loss_coef}
weight_dict['loss_giou'] = args.giou_loss_coef
if args.masks:
    weight_dict["loss_mask"] = args.mask_loss_coef
    weight_dict["loss_dice"] = args.dice_loss_coef
# TODO this is a hack
if args.aux_loss:
    aux_weight_dict = {}
    for i in range(args.dec_layers - 1):
        aux_weight_dict.update({k + f'_{i}': v for k, v in weight_dict.items()})
    weight_dict.update(aux_weight_dict)

losses = ['labels', 'boxes', 'cardinality']
if args.masks:
    losses += ["masks"]
criterion = SetCriterion(num_classes, matcher=matcher, weight_dict=weight_dict,
                         eos_coef=args.eos_coef, losses=losses)

matcher是将预测结果与gt进行匹配的匈牙利算法，weight_dict是各部分loss设置的权重参数，包括分类与回归损失。分类使用的是CE loss，回归包括l1 loss和giou loss。如果包含分割任务，还有mask相关损失函数，另外如果设置了aux_loss，则代表计算decoder中间层预测结果对应的loss。 loss函数的实例化使用SetCriterion进行构建的。

class SetCriterion(nn.Module):
    """ This class computes the loss for DETR.
    The process happens in two steps:
        1) we compute hungarian assignment between ground truth boxes and the outputs of the model
        2) we supervise each pair of matched ground-truth / prediction (supervise class and box)
    """
    def __init__(self, num_classes, matcher, weight_dict, eos_coef, losses):
        """ Create the criterion.
        Parameters:
            num_classes: number of object categories, omitting the special no-object category
            matcher: module able to compute a matching between targets and proposals
            weight_dict: dict containing as key the names of the losses and as values their relative weight.
            eos_coef: relative classification weight applied to the no-object category
            losses: list of all the losses to be applied. See get_loss for list of available losses.
        """
        super().__init__()
        self.num_classes = num_classes
        self.matcher = matcher
        self.weight_dict = weight_dict
        # 针对背景分类的loss权重
        self.eos_coef = eos_coef
        self.losses = losses
        empty_weight = torch.ones(self.num_classes + 1)
        empty_weight[-1] = self.eos_coef
        self.register_buffer('empty_weight', empty_weight)

"""
        '''
        """
    def get_loss(self, loss, outputs, targets, indices, num_boxes, **kwargs):
        loss_map = {
            'labels': self.loss_labels,
            'cardinality': self.loss_cardinality,
            'boxes': self.loss_boxes,
            'masks': self.loss_masks
        }
        assert loss in loss_map, f'do you really want to compute {loss} loss?'
        return loss_map[loss](outputs, targets, indices, num_boxes, **kwargs)

def forward(self, outputs, targets):
        """ This performs the loss computation.
        Parameters:
             outputs: dict of tensors, see the output specification of the model for the format
             targets: list of dicts, such that len(targets) == batch_size.
                      The expected keys in each dict depends on the losses applied, see each loss' doc
        """

outputs_without_aux = {k: v for k, v in outputs.items() if k != 'aux_outputs'}

# Retrieve the matching between the outputs of the last layer and the targets
        # 将预测结果与GT进行匹配，indices是一个与bs长度相等的多元组的list
        # 每个元组为(ind_i,ind_j),前者是匹配的预测预测索引，后者是gt的索引
        indices = self.matcher(outputs_without_aux, targets)

# Compute the average number of target boxes accross all nodes, for normalization purposes
        num_boxes = sum(len(t["labels"]) for t in targets)
        num_boxes = torch.as_tensor([num_boxes], dtype=torch.float, device=next(iter(outputs.values())).device)
        if is_dist_avail_and_initialized():
            torch.distributed.all_reduce(num_boxes)
        num_boxes = torch.clamp(num_boxes / get_world_size(), min=1).item()

# Compute all the requested losses
        # 计算所有相关的损失，其中self.losses = ['labels', 'boxes', 'cardinality']
        losses = {}
        for loss in self.losses:
            losses.update(self.get_loss(loss, outputs, targets, indices, num_boxes))
        """
        '''
        """
        return losses

从forward函数可以看出，首先进行匈牙利匹配的是decoder最后一层的输出，之后再计算匹配后的损失函数包括losses = [‘labels’, ‘boxes’, ‘cardinality’]，具体计算部分可以看get_loss方法中映射的对应计算方法，其中包括self.loss_labels，self.loss_cardinality，self.loss_boxes。

匈牙利匹配

匈牙利算法，在这里用于预测集（prediction set）和GT的匹配，最终匹配方案是选取“loss总和”最小的分配方式。注意，这里计算的loss与损失函数中计算loss并不相同，在这里是用来作为代价cost，cost大小决定匹配程度。

class HungarianMatcher(nn.Module):

def __init__(self, cost_class: float = 1, cost_bbox: float = 1, cost_giou: float = 1):
        """Creates the matcher

Params:
            cost_class: This is the relative weight of the classification error in the matching cost
            cost_bbox: This is the relative weight of the L1 error of the bounding box coordinates in the matching cost
            cost_giou: This is the relative weight of the giou loss of the bounding box in the matching cost
        """
        super().__init__()
        self.cost_class = cost_class
        self.cost_bbox = cost_bbox
        self.cost_giou = cost_giou
        assert cost_class != 0 or cost_bbox != 0 or cost_giou != 0, "all costs cant be 0"

@torch.no_grad()
    def forward(self, outputs, targets):
        bs, num_queries = outputs["pred_logits"].shape[:2]

# We flatten to compute the cost matrices in a batch
        out_prob = outputs["pred_logits"].flatten(0, 1).softmax(-1)  # [batch_size * num_queries, num_classes]
        out_bbox = outputs["pred_boxes"].flatten(0, 1)  # [batch_size * num_queries, 4]

# Also concat the target labels and boxes
        tgt_ids = torch.cat([v["labels"] for v in targets])
        tgt_bbox = torch.cat([v["boxes"] for v in targets])

# Compute the classification cost. Contrary to the loss, we don't use the NLL,
        # but approximate it in 1 - proba[target class].
        # The 1 is a constant that doesn't change the matching, it can be ommitted.
        cost_class = -out_prob[:, tgt_ids]

# Compute the L1 cost between boxes
        cost_bbox = torch.cdist(out_bbox, tgt_bbox, p=1)

# Compute the giou cost betwen boxes
        cost_giou = -generalized_box_iou(box_cxcywh_to_xyxy(out_bbox), box_cxcywh_to_xyxy(tgt_bbox))

# Final cost matrix
        C = self.cost_bbox * cost_bbox + self.cost_class * cost_class + self.cost_giou * cost_giou
        C = C.view(bs, num_queries, -1).cpu()

sizes = [len(v["boxes"]) for v in targets]
        indices = [linear_sum_assignment(c[i]) for i, c in enumerate(C.split(sizes, -1))]
        return [(torch.as_tensor(i, dtype=torch.int64), torch.as_tensor(j, dtype=torch.int64)) for i, j in indices]

从上面可以看到，匈牙利匹配在前向计算过程中，是不需要梯度的。其中分类cost是直接采用1减去预测概率的形式，同时由于1是常数，于是作者甚至连1都省去了，在box上计算了l1和giou两种cost，之后对各部分进行加权求和得到总的cost。匹配方法使用的是 scipy 优化模块中的 linear_sum_assignment()，其输入是二分图的度量矩阵，该方法是计算这个二分图度量矩阵的最小权重分配方式，返回的是匹配方案对应的矩阵行索引和列索引。

至此，DETR所有相关源码均已解读完毕。