Fix csv/json/txt cache dir
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I think this is the way to go but I’m afraid this might be a little slow. I was thinking that we could use a high quality very fast non crypto hash like xxhash for these stuff (hashing data files) Yep good idea, I'll take a look I tested the hashing speed [here](https://colab.research.google.com/drive/1hlhP84kLIHmOzMRQN1h8x10hKWpXXyud?usp=sharing).
I was able to get 8x speed with `xxhashlib` (42ms vs 345ms for 100MiB of data).
What do you think @thomwolf ? I added xxhash and some tests
The cache dir for csv/json/txt datasets was always the same. This is an issue because it should be different depending on the data files provided by the user.

To fix that, I added a line that use the hash of the data files provided by the user to define the cache dir.

This should fix #444 
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