train_test_split error: 'dict' object has no attribute 'deepcopy'
{'login': 'morganmcg1', 'id': 20516801, 'node_id': 'MDQ6VXNlcjIwNTE2ODAx', 'avatar_url': 'https://avatars.githubusercontent.com/u/20516801?v=4', 'gravatar_id': '', 'url': 'https://api.github.com/users/morganmcg1', 'html_url': 'https://github.com/morganmcg1', 'followers_url': 'https://api.github.com/users/morganmcg1/followers', 'following_url': 'https://api.github.com/users/morganmcg1/following{/other_user}', 'gists_url': 'https://api.github.com/users/morganmcg1/gists{/gist_id}', 'starred_url': 'https://api.github.com/users/morganmcg1/starred{/owner}{/repo}', 'subscriptions_url': 'https://api.github.com/users/morganmcg1/subscriptions', 'organizations_url': 'https://api.github.com/users/morganmcg1/orgs', 'repos_url': 'https://api.github.com/users/morganmcg1/repos', 'events_url': 'https://api.github.com/users/morganmcg1/events{/privacy}', 'received_events_url': 'https://api.github.com/users/morganmcg1/received_events', 'type': 'User', 'site_admin': False}
[]
{'login': 'lhoestq', 'id': 42851186, 'node_id': 'MDQ6VXNlcjQyODUxMTg2', 'avatar_url': 'https://avatars.githubusercontent.com/u/42851186?v=4', 'gravatar_id': '', 'url': 'https://api.github.com/users/lhoestq', 'html_url': 'https://github.com/lhoestq', 'followers_url': 'https://api.github.com/users/lhoestq/followers', 'following_url': 'https://api.github.com/users/lhoestq/following{/other_user}', 'gists_url': 'https://api.github.com/users/lhoestq/gists{/gist_id}', 'starred_url': 'https://api.github.com/users/lhoestq/starred{/owner}{/repo}', 'subscriptions_url': 'https://api.github.com/users/lhoestq/subscriptions', 'organizations_url': 'https://api.github.com/users/lhoestq/orgs', 'repos_url': 'https://api.github.com/users/lhoestq/repos', 'events_url': 'https://api.github.com/users/lhoestq/events{/privacy}', 'received_events_url': 'https://api.github.com/users/lhoestq/received_events', 'type': 'User', 'site_admin': False}
[{'login': 'lhoestq', 'id': 42851186, 'node_id': 'MDQ6VXNlcjQyODUxMTg2', 'avatar_url': 'https://avatars.githubusercontent.com/u/42851186?v=4', 'gravatar_id': '', 'url': 'https://api.github.com/users/lhoestq', 'html_url': 'https://github.com/lhoestq', 'followers_url': 'https://api.github.com/users/lhoestq/followers', 'following_url': 'https://api.github.com/users/lhoestq/following{/other_user}', 'gists_url': 'https://api.github.com/users/lhoestq/gists{/gist_id}', 'starred_url': 'https://api.github.com/users/lhoestq/starred{/owner}{/repo}', 'subscriptions_url': 'https://api.github.com/users/lhoestq/subscriptions', 'organizations_url': 'https://api.github.com/users/lhoestq/orgs', 'repos_url': 'https://api.github.com/users/lhoestq/repos', 'events_url': 'https://api.github.com/users/lhoestq/events{/privacy}', 'received_events_url': 'https://api.github.com/users/lhoestq/received_events', 'type': 'User', 'site_admin': False}]
It was fixed in 2ddd18d139d3047c9c3abe96e1e7d05bb360132c.
Could you pull the latest changes from master @morganmcg1 ? Thanks @lhoestq, works fine now!
`train_test_split` is giving me an error when I try and call it:

`'dict' object has no attribute 'deepcopy'`

## To reproduce

```
dataset = load_dataset('glue', 'mrpc', split='train')
dataset = dataset.train_test_split(test_size=0.2)
```

## Full Stacktrace
```
---------------------------------------------------------------------------
AttributeError                            Traceback (most recent call last)
<ipython-input-12-feb740dbec9a> in <module>
      1 dataset = load_dataset('glue', 'mrpc', split='train')
----> 2 dataset = dataset.train_test_split(test_size=0.2)

~/anaconda3/envs/fastai2_me/lib/python3.7/site-packages/nlp/arrow_dataset.py in train_test_split(self, test_size, train_size, shuffle, seed, generator, keep_in_memory, load_from_cache_file, train_cache_file_name, test_cache_file_name, writer_batch_size)
   1032                     "writer_batch_size": writer_batch_size,
   1033                 }
-> 1034                 train_kwargs = cache_kwargs.deepcopy()
   1035                 train_kwargs["split"] = "train"
   1036                 test_kwargs = cache_kwargs.deepcopy()

AttributeError: 'dict' object has no attribute 'deepcopy'
```
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