Loading dataset from local path
Version below works, checked again in the docs, and data_files should be a path.
```
ds = datasets.load_dataset('my_script.py', 
                           data_files='/data/dir/corpus.txt', 
                           cache_dir='.')
```
I'm trying to load a local dataset with the code below

```
ds = datasets.load_dataset('my_script.py', 
                           data_files='corpus.txt', 
                           data_dir='/data/dir', 
                           cache_dir='.')
```
But internally a BuilderConfig is created, which tries to use getmtime on the data_files string, without using data_dir. Is this a bug or am I not using the load_dataset correctly?

https://github.com/huggingface/datasets/blob/bc61954083f74e6460688202e9f77dde2475319c/src/datasets/builder.py#L153