librispeech dataset has to download whole subset when specifing the split to use
Hi! You can use streaming to fetch only a subset of the data:
```python
raw_dataset = load_dataset("librispeech_asr", "clean", split="train.100", streaming=True)
```
Also, we plan to make it possible to download a particular split in the non-streaming mode, but this task is not easy due to how our dataset scripts are structured. Hi,

That's a great help. Thank you very much.
## Describe the bug
librispeech dataset has to download whole subset when specifing the split to use

## Steps to reproduce the bug
see below
# Sample code to reproduce the bug
```
!pip install datasets
from datasets import load_dataset
raw_dataset = load_dataset("librispeech_asr", "clean", split="train.100")
```

## Expected results
The split "train.clean.100" is downloaded.

## Actual results
All four splits in "clean" subset is downloaded.

## Environment info
- `datasets` version: 2.3.2
- Platform: Linux-5.4.188+-x86_64-with-Ubuntu-18.04-bionic
- Python version: 3.7.13
- PyArrow version: 6.0.1
- Pandas version: 1.3.5
