Issue: boolq does not work 

Hi
I am getting this error when trying to load boolq, thanks for your help

ts_boolq_default_0.1.0_2987db1f15deaa19500ae24de560eabeaf1f8ef51df88c0470beeec72943bf11.lock
Traceback (most recent call last):
  File "finetune_t5_trainer.py", line 274, in <module>
    main()
  File "finetune_t5_trainer.py", line 147, in main
    for task in data_args.tasks]
  File "finetune_t5_trainer.py", line 147, in <listcomp>
    for task in data_args.tasks]
  File "/remote/idiap.svm/user.active/rkarimi/dev/ruse/seq2seq/tasks/tasks.py", line 58, in get_dataset
    dataset = self.load_dataset(split=split)
  File "/remote/idiap.svm/user.active/rkarimi/dev/ruse/seq2seq/tasks/tasks.py", line 54, in load_dataset
    return datasets.load_dataset(self.task.name, split=split)
  File "/idiap/user/rkarimi/libs/anaconda3/envs/internship/lib/python3.7/site-packages/datasets/load.py", line 611, in load_dataset
    ignore_verifications=ignore_verifications,
  File "/idiap/user/rkarimi/libs/anaconda3/envs/internship/lib/python3.7/site-packages/datasets/builder.py", line 476, in download_and_prepare
    dl_manager=dl_manager, verify_infos=verify_infos, **download_and_prepare_kwargs
  File "/idiap/user/rkarimi/libs/anaconda3/envs/internship/lib/python3.7/site-packages/datasets/builder.py", line 531, in _download_and_prepare
    split_generators = self._split_generators(dl_manager, **split_generators_kwargs)
  File " /idiap/home/rkarimi/.cache/huggingface/modules/datasets_modules/datasets/boolq/2987db1f15deaa19500ae24de560eabeaf1f8ef51df88c0470beeec72943bf11/boolq.py", line 74, in _split_generators
    downloaded_files = dl_manager.download_custom(urls_to_download, tf.io.gfile.copy)
  File "/idiap/user/rkarimi/libs/anaconda3/envs/internship/lib/python3.7/site-packages/datasets/utils/download_manager.py", line 149, in download_custom
    custom_download(url, path)
  File "/idiap/user/rkarimi/libs/anaconda3/envs/internship/lib/python3.7/site-packages/tensorflow/python/lib/io/file_io.py", line 516, in copy_v2
    compat.path_to_bytes(src), compat.path_to_bytes(dst), overwrite)
tensorflow.python.framework.errors_impl.AlreadyExistsError: file already exists



Comments:
here is the minimal code to reproduce

`datasets>>> datasets.load_dataset("boolq", "train")

the errors

```
`cahce dir  /idiap/temp/rkarimi/cache_home_1/datasets
cahce dir  /idiap/temp/rkarimi/cache_home_1/datasets
Using custom data configuration train
Downloading and preparing dataset boolq/train (download: Unknown size, generated: Unknown size, post-processed: Unknown size, total: Unknown size) to /idiap/temp/rkarimi/cache_home_1/datasets/boolq/train/0.1.0/2987db1f15deaa19500ae24de560eabeaf1f8ef51df88c0470beeec72943bf11...
cahce dir  /idiap/temp/rkarimi/cache_home_1/datasets
cahce dir  /idiap/temp/rkarimi/cache_home_1/datasets/downloads
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/idiap/user/rkarimi/libs/anaconda3/envs/internship/lib/python3.7/site-packages/datasets/load.py", line 611, in load_dataset
    ignore_verifications=ignore_verifications,
  File "/idiap/user/rkarimi/libs/anaconda3/envs/internship/lib/python3.7/site-packages/datasets/builder.py", line 476, in download_and_prepare
    dl_manager=dl_manager, verify_infos=verify_infos, **download_and_prepare_kwargs
  File "/idiap/user/rkarimi/libs/anaconda3/envs/internship/lib/python3.7/site-packages/datasets/builder.py", line 531, in _download_and_prepare
    split_generators = self._split_generators(dl_manager, **split_generators_kwargs)
  File " /idiap/home/rkarimi/.cache/huggingface/modules/datasets_modules/datasets/boolq/2987db1f15deaa19500ae24de560eabeaf1f8ef51df88c0470beeec72943bf11/boolq.py", line 74, in _split_generators
    downloaded_files = dl_manager.download_custom(urls_to_download, tf.io.gfile.copy)
  File "/idiap/user/rkarimi/libs/anaconda3/envs/internship/lib/python3.7/site-packages/datasets/utils/download_manager.py", line 149, in download_custom
    custom_download(url, path)
  File "/idiap/user/rkarimi/libs/anaconda3/envs/internship/lib/python3.7/site-packages/tensorflow/python/lib/io/file_io.py", line 516, in copy_v2
    compat.path_to_bytes(src), compat.path_to_bytes(dst), overwrite)



```

This has been fixed by #881 
this fix will be available in the next release soon.

If you don't want to wait for the release you can actually load the latest version of boolq by specifying `script_version="master"` in `load_dataset`

thank you this solved this issue, for now seems to work, thanks