Differently dataset object saved than it is loaded.
I see now, that I have to use `load_from_disk`, in order to load dataset properly, not `load_dataset`. Why is this behavior split? Why do we need both, `load_dataset` and `load_from_disk`?

Unless answered, I believe this might be helpful for other hf datasets newbies.

Anyway, made a `load_dataset` compatible dataset in a following way. I created a directory, and just copied jsonl there as `train.jsonl/test.jsonl`.
```python
output_folder = os.path.join(args.output_folder, f"{task_meta_type}_{task_type}")
os.makedirs(output_folder, exist_ok=True)
file = f"{task_meta_type}_{task_type}_train.jsonl"
shutil.copy(os.path.join(input_folder, file),
            os.path.join(output_folder, "train.jsonl"))
# now test
file = f"{task_meta_type}_{task_type}_test.jsonl"
shutil.copy(os.path.join(input_folder, file),
            os.path.join(output_folder, "test.jsonl"))
```
 Hi @MFajcik, 

You can find information about save_to_disk/load_from_disk in our docs:
- https://huggingface.co/docs/datasets/v2.16.1/en/process#save
- https://huggingface.co/docs/datasets/v2.16.1/en/package_reference/main_classes#datasets.Dataset.save_to_disk
- https://huggingface.co/docs/datasets/v2.16.1/en/package_reference/main_classes#datasets.Dataset.load_from_disk
### Describe the bug

Differently sized object is saved than it is loaded.

### Steps to reproduce the bug

Hi, I save dataset in a following way:
```
        dataset = load_dataset("json",
                               data_files={
                                   "train": os.path.join(input_folder, f"{task_meta_type}_{task_type}_train.jsonl"),
                                   "test": os.path.join(input_folder, f"{task_meta_type}_{task_type}_test.jsonl")})
        print(os.path.join(output_folder, f"{task_meta_type}_{task_type}"))
        print(f"Length of train dataset: {len(dataset['train'])}")
        print(f"Length of test dataset: {len(dataset['test'])}")
        dataset.save_to_disk(os.path.join(output_folder, f"{task_meta_type}_{task_type}"))
```

this yields output
```
.data/hf_dataset/propaganda_zanr
Length of train dataset: 7642
Length of test dataset: 1000
```

Everything looks fine.

Then I load the dataset
```python
from datasets import load_dataset

dataset_path = ".data/hf_dataset/propaganda_zanr"
dataset = load_dataset(dataset_path)

print(f"Length of train dataset: {len(dataset['train'])}")
print(f"Length of test dataset: {len(dataset['test'])}")

```

this prints
```
Generating train split: 1 examples [00:00, 72.10 examples/s]
Generating test split: 1 examples [00:00, 100.69 examples/s]
Length of train dataset: 1
Length of test dataset: 1
```

I dont' understand :(


### Expected behavior

same object is loaded

### Environment info

datasets==2.16.1