Unify dataset_name and dataset_cls

Currently we are using `dataset_name` in data config, but it is not intuitive since that’s sometimes a dataset class.

1. Remove `dataset_name`, always use `dataset_cls` to make it easy to understand.
2. Support `dataset_cls` to load from path to support a custom `dataset_cls`.
3. Keep the compatibility of `dataset_name` for now, and remove all of them later.

```python
# [Before]
# for supported dataset
dataset_name: OpenMathInstruct-2

# for custom dataset
dataset_name: ResponseDataset
train_data_path:

# [After]
# for supported dataset
dataset_cls: OpenMathInstruct2Dataset

# for custom dataset
dataset_cls: ResponseDataset
train_data_path:
```