load_dataset method returns Unknown split "validation" even if this dir exists
Author: SamSamhuns | Pull request: no | 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)