Keep loading old file even I specify a new file in load_dataset
I used load a file called 'a.csv' by 
```
dataset = load_dataset('csv', data_file='./a.csv')
```
And after a while, I tried to load another csv called 'b.csv'
```
dataset = load_dataset('csv', data_file='./b.csv')
```
However, the new dataset seems to remain the old 'a.csv' and not loading new csv file.

Even worse, after I load a.csv, the load_dataset function keeps loading the 'a.csv' afterward. 

Is this a cache problem?
