fix: handle integer labels in creating labels.txt in export_yoloV8

[alibdz]

The previous code in utils/export_yoloV8.py assumed label values are always strings, causing issues when the model has integer labels (like: 0, 1, 2, ...). This commit adds a check to handle both string and integer labels properly. If the label is an integer, it is converted to a string before writing to the 'labels.txt' file.