Check builder cls default config name in inspect
Fix https://github.com/huggingface/datasets-server/issues/1812

this was causing this issue:

```ipython
In [1]: from datasets import *

In [2]: inspect.get_dataset_config_names("aakanksha/udpos")
Out[2]: ['default']

In [3]: load_dataset_builder("aakanksha/udpos").config.name
Out[3]: 'en'
```