Multithreaded downloads
...for faster dataset download when there are many many small files (e.g. imagefolder, audiofolder)

### Behcnmark

for example on [lhoestq/tmp-images-writer_batch_size](https://hf.co/datasets/lhoestq/tmp-images-writer_batch_size) (128 images)

|    | duration of the download step in `load_dataset()` |
|--| ----------------------------------------------------------------------|
| Before | 58s |
| Now | 3s |

This should fix issues with the Dataset Viewer taking too much time to show up for imagefolder/audiofolder datasets.

### Implementation details

The main change is in the `DownloadManager`:

```diff
- download_func = partial(self._download, download_config=download_config)
+ download_func = partial(self._download_batched, download_config=download_config)
downloaded_path_or_paths = map_nested(
    download_func,
    url_or_urls,
    map_tuple=True,
    num_proc=download_config.num_proc,
    desc="Downloading data files",
+   batched=True,
+   batch_size=-1,
)
```

and `_download_batched` is a multithreaded function.

I only enable multithreading if there are more than 16 files and files are small though, otherwise the progress bar that counts the number of downloaded files is not fluid (updating when a big batch of big files are done downloading). To do so I simply check if the first file is smaller than 20MB.

I also had to tweak `map_nested` to support batching. In particular it slices the data correctly if the user also enables multiprocessing.