label-studio-converter

## Bug Description:
When executing the command **label-studio-converter import yolo -i data -o ls-tasks.json** to generate a JSON file for pre-annotated YOLO detection images, the following folder architecture is assumed:
data:
- images
- labels
- classes.txt
The images folder contains **5537** images, and the labels folder contains an equal number of txt files corresponding to annotations. However, the generated JSON file only contains **3948** images. I've confirmed that each image has a corresponding txt file, and they were generated using a trained YOLOv8 model.

## Environment:
Label Studio latest Docker image

首先，使用所描述的图像、标签和 classes.txt 结构重现 `label-studio-converter import yolo -i data -o ls-tasks.json`，然后检查 converter 如何发现并序列化 YOLO 输入。当生成的 JSON 包含全部 5537 张图像及其对应的标注时，即表示完成。

由索引模型根据 Issue 内容生成。