shuffle does not accept seed 
Hi, did you check the doc on `shuffle`?
https://huggingface.co/docs/datasets/package_reference/main_classes.html?datasets.Dataset.shuffle#datasets.Dataset.shuffle Hi Thomas
thanks for reponse, yes, I did checked it, but this does not work for me please see 

```
(internship) rkarimi@italix17:/idiap/user/rkarimi/dev$ python 
Python 3.7.9 (default, Aug 31 2020, 12:42:55) 
[GCC 7.3.0] :: Anaconda, Inc. on linux
Type "help", "copyright", "credits" or "license" for more information.
>>> import datasets 
2020-12-20 01:48:50.766004: W tensorflow/stream_executor/platform/default/dso_loader.cc:60] Could not load dynamic library 'libcudart.so.11.0'; dlerror: libcudart.so.11.0: cannot open shared object file: No such file or directory
2020-12-20 01:48:50.766029: I tensorflow/stream_executor/cuda/cudart_stub.cc:29] Ignore above cudart dlerror if you do not have a GPU set up on your machine.
>>> data = datasets.load_dataset("scitail", "snli_format")
cahce dir  /idiap/temp/rkarimi/cache_home_1/datasets
cahce dir  /idiap/temp/rkarimi/cache_home_1/datasets
Reusing dataset scitail (/idiap/temp/rkarimi/cache_home_1/datasets/scitail/snli_format/1.1.0/fd8ccdfc3134ce86eb4ef10ba7f21ee2a125c946e26bb1dd3625fe74f48d3b90)
>>> data.shuffle(seed=2)
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: shuffle() got an unexpected keyword argument 'seed'

```

datasets version
`datasets                  1.1.2                     <pip>
`
 Thanks for reporting ! 

Indeed it looks like an issue with `suffle` on `DatasetDict`. We're going to fix that.
In the meantime you can shuffle each split (train, validation, test) separately:
```python
shuffled_train_dataset = data["train"].shuffle(seed=42)
```

Hi
I need to shuffle the dataset, but this needs to be based on epoch+seed to be consistent across the cores, when I pass seed to shuffle, this does not accept seed, could you assist me with this? thanks  @lhoestq
 