add eval dataset support

[codemayq]

2. merge load dataset and split dataset function

What does this PR do?

1. add custom eval dataset support during training
2. reconstruct the load dataset and split dataset functions

fix #2290
fix #3974

Before submitting

• Did you read the contributor guideline?
• Did you write any new necessary tests?

[hiyouga]

LGTM

[dqgdqg]

Thanks for the valuable update. But now it is a little bit confusing.

Could you please give an example of **.yaml configuration to specify different train and val datasets instead of val_size?

[hiyouga]

Thanks for the valuable update. But now it is a little bit confusing.

Could you please give an example of **.yaml configuration to specify different train and val datasets instead of val_size?

remove val_size and add eval_dataset

[dqgdqg]

It works. Thanks.

[Wolverhampton0]

还是报错：ValueError: Some keys are not used by the HfArgumentParser: ['eval_dataset']是为何？需要更新环境吗？