load_dataset() should load all subsets, if no specific subset is specified
### Feature request

Currently load_dataset() is forcing users to specify a subset. Example

`from datasets import load_dataset
dataset = load_dataset("m-a-p/COIG-CQIA")`

```---------------------------------------------------------------------------
ValueError                                Traceback (most recent call last)
[<ipython-input-10-c0cb49385da6>](https://localhost:8080/#) in <cell line: 2>()
      1 from datasets import load_dataset
----> 2 dataset = load_dataset("m-a-p/COIG-CQIA")

3 frames
[/usr/local/lib/python3.10/dist-packages/datasets/builder.py](https://localhost:8080/#) in _create_builder_config(self, config_name, custom_features, **config_kwargs)
    582                     if not config_kwargs:
    583                         example_of_usage = f"load_dataset('{self.dataset_name}', '{self.BUILDER_CONFIGS[0].name}')"
--> 584                         raise ValueError(
    585                             "Config name is missing."
    586                             f"\nPlease pick one among the available configs: {list(self.builder_configs.keys())}"

ValueError: Config name is missing.
Please pick one among the available configs: ['chinese_traditional', 'coig_pc', 'exam', 'finance', 'douban', 'human_value', 'logi_qa', 'ruozhiba', 'segmentfault', 'wiki', 'wikihow', 'xhs', 'zhihu']
Example of usage:
	`load_dataset('coig-cqia', 'chinese_traditional')`
```
This means a dataset cannot contain all the subsets at the same time. I guess one workaround is to manually specify the subset files like in [here](https://huggingface.co/datasets/m-a-p/COIG-CQIA/discussions/1#658698b44bb41498f75c5622), which is clumsy.




### Motivation

Ideally, if not subset is specified, the API should just try to load all subsets. This makes it much easier to handle datasets w/ subsets.

### Your contribution

Not sure since I'm not familiar w/ the lib src.