Length of table does not accurately reflect the split
As already replied by @lhoestq (private channel):
> `.train_test_split` (as well as `.shard`, `.select`) doesn't create a new arrow table to save time and disk space. Instead, it uses an indices mapping on top of the table that locate which examples are part of train or test. This is an optimization that we don't plan to "fix", so I'm closing this issue.
### Describe the bug

I load a Huggingface Dataset and do `train_test_split`. I'm expecting the underlying table for the dataset to also be split, but it's not.

### Steps to reproduce the bug

![image](https://github.com/huggingface/datasets/assets/8068268/83e5768f-8b4c-422a-945c-832a7585afff)


### Expected behavior

The expected behavior is when `len(hf_dataset["train"].data)` should match the length of the train split, and not be the entire unsplit dataset.

### Environment info

datasets 2.10.1
python 3.10.11