Q: Memory release when removing the column?
Hi ! Datasets are memory mapped from your disk, i.e. they're not loaded in RAM. This is possible thanks to the Arrow data format.

Therefore the column you remove is not in RAM, so removing it doesn't cause the RAM to decrease. Thanks for the explanation! @lhoestq 
I wonder since it is memory mapped, can we reduce or remove this memory map? Yes you can `del common_voice` for example or wait for it to be garbage collected
### Describe the bug

How do I release memory when I use methods like `.remove_columns()` or `clear()` in notebooks?
```python
from datasets import load_dataset
common_voice = load_dataset("mozilla-foundation/common_voice_11_0", "ja", use_auth_token=True)
# check memory -> RAM Used (GB):  0.704 / Total (GB)  33.670
common_voice = common_voice.remove_columns(column_names=common_voice.column_names['train'])
common_voice.clear()
# check memory -> RAM Used (GB):  0.705 / Total (GB)  33.670
```

I tried `gc.collect()` but did not help

### Steps to reproduce the bug

1. load dataset
2. remove all the columns
3. check memory is reduced or not


[link to reproduce](https://www.kaggle.com/code/bayartsogtya/huggingface-dataset-memory-issue/notebook?scriptVersionId=110630567)

### Expected behavior

Memory released when I remove the column

### Environment info

- `datasets` version: 2.1.0
- Platform: Linux-5.15.65+-x86_64-with-debian-bullseye-sid
- Python version: 3.7.12
- PyArrow version: 8.0.0
- Pandas version: 1.3.5