Bug: mosaic_prob is no 1 or 0, AttributeError: 'int' object has no attribute 'numel'

[OceanWong1991]

Hi, @FateScript, recently, when I test the Yolox, I find when you set 0<mosaic_prob<1, we may have this bug:

numel = sum(x.numel() for x in batch)
AttributeError: 'int' object has no attribute 'numel'

I debug it find that:

When we ConcatDataset, The data type should be same, So I think we should just return idx, because when I read the source code the img_id is totally same with idx.