Resolve data_files by split name
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Awesome @lhoestq I like the proposal and it works great on my JSON community dataset. Here is the [log](https://gist.github.com/vblagoje/714babc325bcbdd5de579fd8e1648892).  From my discussion with @borisdayma it would be more general the files match if their paths contains the split name - not only if the filename contains the split name. For example for a dataset like this:
```
train/
└── data.csv
test/
└── data.csv
```

But IMO the default should be 
```
data/
├── train.csv
└── test.csv
```
because it allows people to have other directories if they have different subsets of their data (different configurations, not splits) I just created a PR for this at https://github.com/huggingface/datasets/pull/3221, let me know what you think :)
This issue is about discussing the default behavior when someone loads a dataset that consists in data files. For example:
```python
load_dataset("lhoestq/demo1")
```
should return two splits "train" and "test" since the dataset repostiory is like
```
data/
├── train.csv
└── test.csv
```
Currently it returns only one split "train" which contains the data of both files


I started playing with this idea on this branch btw: `resolve-data_files-by-split-name`
Basically the idea is that if you named you data files after split names then the default pattern is
```python
{
    "train": ["*train*"],
    "test": ["*test*"],
    "validation": ["*dev*", "valid"],
}
```
otherwise it's
```python
{
    "train": ["*"]
}
```

Let me know what you think !
cc @albertvillanova @LysandreJik @vblagoje 
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