本节是 DETR流程及 构建backbone和position embedding 相关部分的代码解析 

一、DETR代码流程：

STEP 1: Create model and criterion    #构建模型和标准

STEP 2: Create train and val dataloader  #构建训练和测试数据集

STEP 3: Define lr_scheduler  #定义学习策略

STEP 4: Define optimizer   #定义优化器

STEP 5: Load pretrained model or load resume model and optimizer states 加载预训练模型

STEP 6: Validation 

STEP 7: Start training and validation 

STEP 8：Model save

STEP 1: Create model and criterion    #构建模型和标准

 model, criterion, postprocessors = build_detr(config)

def build_detr(config):

    """ build detr model from config"""

    # 1. build backbone with position embedding
    backbone = build_backbone(config)

    # 2. build transformer (encoders and decoders)
    transformer = build_tra