load_dataset method returns Unknown split "validation" even if this dir exists
State: closed

## Describe the bug
The `datasets.load_dataset` returns a `ValueError: Unknown split "validation". Should be one of ['train', 'test'].` when running `load_dataset(local_data_dir_path, split="validation")` even if the `validation` sub-directory exists in the local data path.

The data directories are as follows and attached to this issue:
```
test_data1
              |_ train
                  |_ 1012.png
                  |_ metadata.jsonl
                  ...
              |_ test
                  ...
              |_ validation
                  |_ 234.png
                  |_ metadata.jsonl
                  ...
test_data2
              |_ train
                  |_ train_1012.png
                  |_ metadata.jsonl
                  ...
              |_ test
                  ...
              |_ validation
                  |_ val_234.png
                  |_ metadata.jsonl
                  ...
```

They contain the same image files and `metadata.jsonl` but the images in `test_data2` have the split names prepended i.e.
`train_1012.png, val_234.png` and the images in  `test_data1` do not have the split names prepended to the image names i.e. `1012.png, 234.png`

I actually saw in another issue `val` was not recognized as a split name but here I would expect the files to take the split from the parent directory name i.e. val should become part of the validation split?

## Steps to reproduce the bug
```python
import datasets
datasets.logging.set_verbosity_error()
from datasets import load_dataset, get_dataset_split_names


# the following only finds train, validation and test splits correctly
path = "./test_data1"
print("######################", get_dataset_split_names(path), "######################")

dataset_list = []
for spt in ["train", "test", "validation"]:
    dataset = load_dataset(path, split=spt)
    dataset_list.append(dataset)


# the following only finds train and test splits
path = "./test_data2"
print("######################", get_dataset_split_names(path), "######################")

dataset_list = []
for spt in ["train", "test", "validation"]:
    dataset = load_dataset(path, split=spt)
    dataset_list.append(dataset)
```


## Expected results
```
###################### ['train', 'test', 'validation'] ######################
###################### ['train', 'test', 'validation'] ######################
```

## Actual results
```
Traceback (most recent call last):
  File "test_data_loader.py", line 11, in <module>

    dataset = load_dataset(path, split=spt)
  File "/home/venv/lib/python3.8/site-packages/datasets/load.py", line 1758, in load_dataset
    ds = builder_instance.as_dataset(split=split, ignore_verifications=ignore_verifications, in_memory=keep_in_memory)
  File "/home/venv/lib/python3.8/site-packages/datasets/builder.py", line 893, in as_dataset
    datasets = map_nested(
  File "/home/venv/lib/python3.8/site-packages/datasets/utils/py_utils.py", line 385, in map_nested
    return function(data_struct)
  File "/home/venv/lib/python3.8/site-packages/datasets/builder.py", line 924, in _build_single_dataset
    ds = self._as_dataset(
  File "/home/venv/lib/python3.8/site-packages/datasets/builder.py", line 993, in _as_dataset
    dataset_kwargs = ArrowReader(self._cache_dir, self.info).read(
  File "/home/venv/lib/python3.8/site-packages/datasets/arrow_reader.py", line 211, in read
    files = self.get_file_instructions(name, instructions, split_infos)
  File "/home/venv/lib/python3.8/site-packages/datasets/arrow_reader.py", line 184, in get_file_instructions
    file_instructions = make_file_instructions(
  File "/home/venv/lib/python3.8/site-packages/datasets/arrow_reader.py", line 107, in make_file_instructions
    absolute_instructions = instruction.to_absolute(name2len)
  File "/home/venv/lib/python3.8/site-packages/datasets/arrow_reader.py", line 616, in to_absolute
    return [_rel_to_abs_instr(rel_instr, name2len) for rel_instr in self._relative_instructions]
  File "/home/venv/lib/python3.8/site-packages/datasets/arrow_reader.py", line 616, in <listcomp>
    return [_rel_to_abs_instr(rel_instr, name2len) for rel_instr in self._relative_instructions]
  File "/home/venv/lib/python3.8/site-packages/datasets/arrow_reader.py", line 433, in _rel_to_abs_instr
    raise ValueError(f'Unknown split "{split}". Should be one of {list(name2len)}.')
ValueError: Unknown split "validation". Should be one of ['train', 'test'].
```

## Environment info
<!-- You can run the command `datasets-cli env` and copy-and-paste its output below. -->
- `datasets` version:
- Platform: Linux Ubuntu 18.04
- Python version: 3.8.12
- PyArrow version: 9.0.0

Data files

[test_data1.zip](https://github.com/huggingface/datasets/files/9424463/test_data1.zip)
[test_data2.zip](https://github.com/huggingface/datasets/files/9424468/test_data2.zip)



I don't know the main problem but it looks like, it is ignoring the last directory in your case. So, create a directory called 'zzz' in the same folder as train, validation and test. if it doesn't work, create a directory called "aaa". It worked for me.


@SamSamhuns could you please try to load it with the current main-branch version of `datasets`? I suppose the problem is that it tries to get splits names from filenames in this case, ignoring directories names,  but `val` wasn't in keywords at that time, but it was fixed recently in this PR  https://github.com/huggingface/datasets/pull/4844. 

I have a similar problem.
When I try to create `data_infos.json` using `datasets-cli test Peter.py --save_infos --all_configs` I get an error:
`ValueError: Unknown split "test". Should be one of ['train'].`

The `data_infos.json` is created perfectly fine when I use only one split - `datasets.Split.TRAIN`

@polinaeterna  Could you help here please?

You can find the code here: https://huggingface.co/datasets/sberbank-ai/Peter/tree/add_splits (add_splits branch)

@skalinin It seems the `dataset_infos.json` of your dataset is missing the info on the test split (and `datasets-cli` doesn't ignore the cached infos at the moment, which is a known bug), so your issue is not related to this one. I think you can fix your issue by deleting all the cached `dataset_infos.json` (in the local repo and in `~/.cache/huggingface/modules`) before running the `datasets-cli test` command. Let us know if that doesn't help, and I can try to generate it myself.

This code indeed behaves as expected on `main`. But suppose the `val_234.png` is renamed to some other value not containing one of [these](https://github.com/huggingface/datasets/blob/38c8c725f3996ff1ff03f6fd461aa6d645321034/src/datasets/data_files.py#L31) keywords, in that case, this issue becomes relevant again because the real cause of it is the order in which we check the predefined split patterns to assign data files to each split - first we assign data files based on filenames, and only if this fails meaning not a single split found (`val` is not recognized here in the older versions of `datasets`, which results in an empty `validation` split), do we assign based on directory names.

@polinaeterna @lhoestq Perhaps one way to fix this would be to swap the [order](https://github.com/huggingface/datasets/blob/38c8c725f3996ff1ff03f6fd461aa6d645321034/src/datasets/data_files.py#L78-L79) of the patterns if `data_dir` is specified (or if `load_dataset(data_dir)` is called)? 

> @polinaeterna @lhoestq Perhaps one way to fix this would be to swap the [order](https://github.com/huggingface/datasets/blob/38c8c725f3996ff1ff03f6fd461aa6d645321034/src/datasets/data_files.py#L78-L79) of the patterns if data_dir is specified (or if load_dataset(data_dir) is called)?

yes that makes sense !

Looks like the `val/validation` dir name issue is fixed with the current main-branch version of the `datasets` repository. 

> @polinaeterna @lhoestq Perhaps one way to fix this would be to swap the [order](https://github.com/huggingface/datasets/blob/38c8c725f3996ff1ff03f6fd461aa6d645321034/src/datasets/data_files.py#L78-L79) of the patterns if data_dir is specified (or if load_dataset(data_dir) is called)?

I agree with this as well. I would expect higher precedence to the directory name over the file name. Right now if I place a single file named `train_00001.jpg` under the `validation` directory, `load_dataset` cannot find the validation split.

Thanks for the reply

I've created a separate [issue](https://github.com/huggingface/datasets/issues/4982#issue-1375604693) for my problem.

> @polinaeterna @lhoestq Perhaps one way to fix this would be to swap the [order](https://github.com/huggingface/datasets/blob/38c8c725f3996ff1ff03f6fd461aa6d645321034/src/datasets/data_files.py#L78-L79) of the patterns if data_dir is specified (or if load_dataset(data_dir) is called)?

Sounds good to me! opened a PR: https://github.com/huggingface/datasets/pull/4985

Hi there @polinaeterna @mariosasko ! I have installed 5.2.3.dev0, which should have this fix. Unfortunately, I am still getting the error:
`ValueError: Unknown split "validation". Should be one of ['train'].` When I call `load_dataset("csv", data_files=files, split=split)`

Any help would be greatly appreciated!

hi @shaneacton ! could you please show your dataset structure?

Hi there @polinaeterna . My local CSV files are stored as follows:
binding:
---------- tune.csv
---------- public_data:
--------------------------- train.csv

`self.list_shards(split)` sucessfully finds the relevant data files

@shaneacton do you have `validation.csv`/`val.csv`/`valid.csv`/`dev.csv` file in your data folder? I can't find it in the structure you provided

@polinaeterna no, does the name of the split need to match the name of the file exactly?

But my train file is not actually named 'train.py' its called 'XXXXXXXXX_train_XXXXXXXX.csv'
And the code works fine for train, but fails for validation.
Does the file name need to _contain_ the split name?

@shaneacton what files do you expect to be included in "validation" split? yes, you should somehow indicate that a file belongs to a certain split - either by including split name in a filename or by putting it into a folder with split name, you can also check out [this documentation page](https://huggingface.co/docs/datasets/main/en/repository_structure) :)
by default all the data goes to a single `train` split

@polinaeterna I have specified my train/test/tune files via the `split_to_filepattern` argument when initialising my `FileDataSource` class.  This is how `list_shards` is able to find the right files.
After your last message, I have tried renaminig my data files to simply `train.csv` and `validation.csv`, however I am still getting the same error: `Unknown split "validation". Should be one of ['train']`

@polinaeterna I have solved the issue. The solution was to call:
`load_dataset("csv", data_files={split: files}, split=split)`

For me it resolved by adding the verification_mode param:
```
  imdb_ds = load_dataset(
      "imdb", verification_mode="no_checks"
  )
  imdb_ds
```