caption_dropout_rate should be 0 in validation dataset

[kohya-ss]

I found this while implementing #2291.

[kohya-ss]

Hmm, this doesn't seem to work properly. When caption_dropout_rate is greater than 0, all items in the batch end up being dropped. Fixing #2292 will also fix this problem, but it might be better to investigate why the batch items in the validation dataset is being cached.