`DatasetBuilder._split_generators` incomplete type annotation
Good catch! Feel free to open a PR with the suggested fix :). There is also the [`MockDownloadManager`](https://github.com/JonasLoos/datasets/blob/main/src/datasets/download/mock_download_manager.py#L33), which seems like it might get passed here too. However, to me, it doesn't really seem relevant to the users of the datasets library, so I would just ignore it. What do you think, @mariosasko? The API (`dummy_data` CLI command ) that uses the `MockDownloadManager` has been deprecated, so ignoring it sounds good!
### Describe the bug

The [`DatasetBuilder._split_generators`](https://github.com/huggingface/datasets/blob/0f27d7b77c73412cfc50b24354bfd7a3e838202f/src/datasets/builder.py#L1449) function has currently the following signature:

```python
class DatasetBuilder:
    def _split_generators(self, dl_manager: DownloadManager):
        ...
```

However, the `dl_manager` argument can also be of type [`StreamingDownloadManager`](https://github.com/huggingface/datasets/blob/0f27d7b77c73412cfc50b24354bfd7a3e838202f/src/datasets/download/streaming_download_manager.py#L962), which has different functionality. For example, the `download` function doesn't download, but rather just returns the given url(s). 

I suggest changing the function signature to:

```python
class DatasetBuilder:
    def _split_generators(self, dl_manager: Union[DownloadManager, StreamingDownloadManager]):
        ...
```

and also adjust the docstring accordingly.

I would like to create a Pull Request to fix this, and have the following questions:
* Are there also other options than `DownloadManager`, and `StreamingDownloadManager`?
* Should this also be changed in other functions?

### Steps to reproduce the bug

Minimal example to print the different class names:

```python
import tempfile
from datasets import load_dataset

example = b'''
from datasets import GeneratorBasedBuilder, DatasetInfo, Features, Value, SplitGenerator

class Test(GeneratorBasedBuilder):
    def _info(self):
        return DatasetInfo(features=Features({"x": Value("int64")}))
    def _split_generators(self, dl_manager):
        print(type(dl_manager))
        return [SplitGenerator('test')]
    def _generate_examples(self):
        yield 0, {'x': 42}
'''

with tempfile.NamedTemporaryFile(suffix='.py') as f:
    f.write(example)
    f.flush()
    load_dataset(f.name, streaming=False)
    load_dataset(f.name, streaming=True)
```

### Expected behavior

complete type annotations

### Environment info

/