Issue: integrate `load_from_disk` into `load_dataset` 

**Is your feature request related to a problem? Please describe.**

Is it possible to make `load_dataset` more universal similar to `from_pretrained` in `transformers` so that it can handle the hub, and the local path datasets of all supported types?

Currently one has to choose a different loader depending on how the dataset has been created.

e.g. this won't work:

```
$ git clone https://huggingface.co/datasets/severo/test-parquet
$ python -c 'from datasets import load_dataset; ds=load_dataset("test-parquet"); \
ds.save_to_disk("my_dataset"); load_dataset("my_dataset")'

[...]

Traceback (most recent call last):
  File "<string>", line 1, in <module>
  File "/home/stas/anaconda3/envs/py38-pt112/lib/python3.8/site-packages/datasets/load.py", line 1746, in load_dataset
    builder_instance.download_and_prepare(
  File "/home/stas/anaconda3/envs/py38-pt112/lib/python3.8/site-packages/datasets/builder.py", line 704, in download_and_prepare
    self._download_and_prepare(
  File "/home/stas/anaconda3/envs/py38-pt112/lib/python3.8/site-packages/datasets/builder.py", line 793, in _download_and_prepare
    self._prepare_split(split_generator, **prepare_split_kwargs)
  File "/home/stas/anaconda3/envs/py38-pt112/lib/python3.8/site-packages/datasets/builder.py", line 1277, in _prepare_split
    writer.write_table(table)
  File "/home/stas/anaconda3/envs/py38-pt112/lib/python3.8/site-packages/datasets/arrow_writer.py", line 524, in write_table
    pa_table = table_cast(pa_table, self._schema)
  File "/home/stas/anaconda3/envs/py38-pt112/lib/python3.8/site-packages/datasets/table.py", line 2005, in table_cast
    return cast_table_to_schema(table, schema)
  File "/home/stas/anaconda3/envs/py38-pt112/lib/python3.8/site-packages/datasets/table.py", line 1968, in cast_table_to_schema
    raise ValueError(f"Couldn't cast\n{table.schema}\nto\n{features}\nbecause column names don't match")
ValueError: Couldn't cast
_data_files: list<item: struct<filename: string>>
  child 0, item: struct<filename: string>
      child 0, filename: string
```

both times the dataset is being loaded from disk. Why does it fail the second time?

Why can't `save_to_disk` generate a dataset that can be immediately loaded by `load_dataset`?

e.g. the simplest hack would be to have `save_to_disk` add some flag to the saved dataset, that tells `load_dataset` to internally call `load_from_disk`. like having `save_to_disk` create a `load_me_with_load_from_disk.txt` file ;) and `load_dataset` will support that feature from saved datasets from new `datasets` versions. The old ones will still need to use `load_from_disk` explicitly. Unless the flag is not needed and one can immediately tell by looking at the saved dataset that it was saved via `save_to_disk` and thus use `load_from_disk` internally.

The use-case is defining a simple API where the user only ever needs to pass a `dataset_name_or_path` and it will always just work. Currently one needs to manually add additional switches telling the system whether to use one loading method or the other which works but it's not smooth.

Thank you!

Comments:
I agree the situation is not ideal and it would be awesome to use `load_dataset` to reload a dataset saved locally !

For context:

- `load_dataset` works in three steps: download the dataset, then prepare it as an arrow dataset, and finally return a memory mapped arrow dataset. In particular it creates a cache directory to store the arrow data and the subsequent cache files for `map`.

- `load_from_disk` directly returns a memory mapped dataset from the arrow file (similar to `Dataset.from_file`). It doesn't create a cache diretory, instead all the subsequent `map` calls write in the same directory as the original data. 

If we want to keep the download_and_prepare step for consistency, it would unnecessarily copy the arrow data into the datasets cache. On the other hand if we don't do this step, the cache directory doesn't exist which is inconsistent.

I'm curious, what would you expect to happen in this situation ?

Thank you for the detailed breakdown, @lhoestq 

> I'm curious, what would you expect to happen in this situation ?

1. the simplest solution is to add a flag to the dataset saved by `save_to_disk` and have `load_dataset` check that flag - if it's set simply switch control to `load_from_disk` behind the scenes. So `load_dataset` detects it's a local filesystem, looks inside to see whether it's something it can cache or whether it should use it directly as is and continues accordingly with one of the 2 dataset-type specific APIs.

2. the more evolved solution is to look at a dataset produced by `save_to_disk` as a remote resource like hub. So the first time `load_dataset` sees it, it'll take a fingerprint and create a normal cached dataset. On subsequent uses it'll again discover it as a remote resource, validate that it has it cached via the fingerprint and serve as a normal dataset. 

As you said the cons of approach 2 is that if the dataset is huge it'll make 2 copies on the same machine. So it's possible that both approaches can be integrated. Say if `save_to_disc(do_not_cache=True)` is passed it'll use solution 1, otherwise solution 2. or could even symlink the huge arrow files to the cache instead? or perhaps it's more intuitive to use `load_dataset(do_not_cache=True)` instead. So that one can choose whether to make a cached copy or not for the locally saved dataset. i.e. a simple at use point user control.

Surely there are other ways to handle it, this is just one possibility.


I think the simplest is to always memory map the local file without copy, but still have a cached directory in the cache at `~/.cache/huggingface` instead of saving `map` results next to the original data.

In practice we can even use symlinks if it makes the implementation simpler

Yes, so that you always have the cached entry for any dataset, but the "payload" doesn't have to be physically in the cache if it's already on the local filesystem. As you said a symlink will do. 

Any updates?

We haven't had the bandwidth to implement this so far. Let me know if you'd be interested in contributing this feature :)

@lhoestq I can jump into that. What I don't like is having functions with many parameters input. Even though they are optional, it's always harder to reason about and test such cases.
If there are more features worth to work on, feel free to ping me. It's a lot of fun to help :smile: 

Thanks a lot for your help @mariusz-jachimowicz-83 :)

I think as a first step we could implement an Arrow dataset builder to be able to load and stream Arrow datasets locally or from Hugging Face. Maybe something similar to the Parquet builder at [src/datasets/packaged_modules/parquet/parquet.py](https://github.com/huggingface/datasets/blob/main/src/datasets/packaged_modules/parquet/parquet.py) ?

And we can deal with the disk space optimization as a second step. What do you think ?

(this issue is also related to https://github.com/huggingface/datasets/issues/3035)

@lhoestq I made a PR based on suggestion https://github.com/huggingface/datasets/pull/5944. Could you please review it?

@lhoestq Let me know if you have further recommendations or anything that you would like to add but you don't have bandwith for. 

Any update on this issue? It makes existing scripts and examples fall flat when provided with a customized/preprocessed dataset saved to disk.

This would be a really useful in terms of user experience. 

Is there any update on this? This would improves the clarity and consistency of the implementations.

Not yet ! Though we do have an Arrow loader in `load_dataset` now, so the remaining items are:

1. update `load_dataset()` to support the old `save_to_disk()` structure with a Warning message that it's not the structure it generally uses and it's enabled for compatibility purposes

(Q: `load_dataset()` works using a cache that contains cached Arrow files of any dataset, so if the dataset is already in Arrow we can optionally make it symlink the files in the cache instead of copying them ? for consistency I would still copy the data. Especially for cases where the dataset location is on a slow disk, it can be better to copy the data once to the fast cache)

2. update `save_to_disk()` to export in a `load_dataset()` compatible structure