Resolve data_files by split name
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)
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 
Resolve data_files by split name 
 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  
 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)