load_dataset processing failed with OS error after downloading a dataset
Hi ! It looks like an issue with pytorch.

Could you try to run `import torch` and see if it raises an error ? > Hi ! It looks like an issue with pytorch.
> 
> Could you try to run `import torch` and see if it raises an error ?

It works. Thank you!
## Describe the bug
After downloading a dataset like opus100, there is a bug that 
OSError: Cannot find data file.
Original error:
dlopen: cannot load any more object with static TLS

## Steps to reproduce the bug
```python
from datasets import load_dataset
this_dataset = load_dataset('opus100', 'af-en')
```

## Expected results
there is no error when running load_dataset.

## Actual results
Specify the actual results or traceback.

Traceback (most recent call last):
  File "/home/anaconda3/lib/python3.6/site-packages/datasets/builder.py", line 652, in _download_and_prep
    self._prepare_split(split_generator, **prepare_split_kwargs)
  File "/home/anaconda3/lib/python3.6/site-packages/datasets/builder.py", line 989, in _prepare_split
    example = self.info.features.encode_example(record)
  File "/home/anaconda3/lib/python3.6/site-packages/datasets/features.py", line 952, in encode_example
    example = cast_to_python_objects(example)
  File "/home/anaconda3/lib/python3.6/site-packages/datasets/features.py", line 219, in cast_to_python_ob
    return _cast_to_python_objects(obj)[0]
  File "/home/anaconda3/lib/python3.6/site-packages/datasets/features.py", line 165, in _cast_to_python_o
    import torch
  File "/home/anaconda3/lib/python3.6/site-packages/torch/__init__.py", line 188, in <module>
    _load_global_deps()
  File "/home/anaconda3/lib/python3.6/site-packages/torch/__init__.py", line 141, in _load_global_deps
    ctypes.CDLL(lib_path, mode=ctypes.RTLD_GLOBAL)
  File "/home/anaconda3/lib/python3.6/ctypes/__init__.py", line 348, in __init__
    self._handle = _dlopen(self._name, mode)
OSError: dlopen: cannot load any more object with static TLS

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
  File "download_hub_opus100.py", line 9, in <module>
    this_dataset = load_dataset('opus100', language_pair)
  File "/home/anaconda3/lib/python3.6/site-packages/datasets/load.py", line 748, in load_dataset
    use_auth_token=use_auth_token,
  File "/home/anaconda3/lib/python3.6/site-packages/datasets/builder.py", line 575, in download_and_prepa
    dl_manager=dl_manager, verify_infos=verify_infos, **download_and_prepare_kwargs
  File "/home/anaconda3/lib/python3.6/site-packages/datasets/builder.py", line 658, in _download_and_prep
    + str(e)
OSError: Cannot find data file.
Original error:
dlopen: cannot load any more object with static TLS


## Environment info
- `datasets` version: 1.8.0
- Platform: Linux-3.13.0-32-generic-x86_64-with-debian-jessie-sid
- Python version: 3.6.6
- PyArrow version: 3.0.0


