hotfix(dataloader): need specific len otherwise it may stuck of out of index.

[Kin-Zhang]

No error info will pop up on the previous version.

Need specific the len in the dataset otherwise it will out of index.

[Kin-Zhang]

btw, another point for this sentence in README:
You may achieve better model performance by adjusting hyper-parameters such as the learning rate and increasing the batch size.

I think we need be careful about the batch size since the diffsynth only support bz=1 right now, some discussion can be found Wan2.1 finetuning script seems to only support bs = 1