datasets generate large arrow file
Hi ! The cache files are the results of all the transforms you applied to the dataset using `map` for example.
Did you run a transform that could potentially blow up the size of the dataset ? @lhoestq,
I don't remember, but I can't imagine what kind of transform may generate data that grow over 200 times in size. 
I think maybe it doesn' matter, it's just cache after all.
Checking the large file in disk, and found the large cache file in the cifar10 data directory:

![image](https://user-images.githubusercontent.com/18533904/186830449-ba96cdeb-0fe8-4543-994d-2abe7145933f.png)

As we know, the size of cifar10 dataset is ~130MB, but the cache file has almost 30GB size, there may be some problems here.