Rename DatasetBuilder config_name
This PR renames the DatasetBuilder keyword argument `name` to `config_name` so that:
- it avoids confusion with the attribute `DatasetBuilder.name`, which is different
- it aligns with the Dataset property name `config_name`, defined in `DatasetInfoMixin.config_name`

Other simpler possibility could be to rename it to just `config` instead.

Please note I have only renamed this argument of DatasetBuilder because I think this refactoring has a low impact on users: we can assume this is not a public facing parameter, but private or related to the inners of our library.

It would have a  major impact to rename it also in:
- load_dataset
- load_dataset_builder: although this could also be assumed as inners...
- in our CLI commands

Besides the naming of `name`, I also find really confusing the naming of `path` in `load_dataset`. IMHO, they should have a more simpler and precise meaning (currently, they are too vague). I would propose (maybe for next major release):
```
load_dataset(dataset, config,...
```
instead of
```
load_dataset(path, name,...
```
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