Couldn't reach certain URLs and for the ones that can be reached, code just blocks after downloading. 
 Hi,

I'm following the [quick tour](https://huggingface.co/nlp/quicktour.html) and tried to load the glue dataset:
```
>>> from nlp import load_dataset
>>> dataset = load_dataset('glue', 'mrpc', split='train')
```

However, this ran into a `ConnectionError` saying it could not reach the URL (just pasting the last few lines):
```

/net/vaosl01/opt/NFS/su0/miniconda3/envs/hf/lib/python3.7/site-packages/nlp/utils/file_utils.py in get_from_cache(url, cache_dir, force_download, proxies, etag_timeout, resume_download, user_agent, local_files_only)
    354                 " to False."
    355             )
--> 356         raise ConnectionError("Couldn't reach {}".format(url))
    357 
    358     # From now on, connected is True.

ConnectionError: Couldn't reach https://firebasestorage.googleapis.com/v0/b/mtl-sentence-representations.appspot.com/o/data%2Fmrpc_dev_ids.tsv?alt=media&token=ec5c0836-31d5-48f4-b431-7480817f1adc
```

I tried glue with cola and sst2. I got the same error, just instead of mrpc in the URL, it was replaced with cola and sst2.

Since this was not working, I thought I'll try another dataset. So I tried downloading the imdb dataset:
```
ds = load_dataset('imdb', split='train')
```
This downloads the data, but it just blocks after that:
```
Downloading: 100%|████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████| 4.56k/4.56k [00:00<00:00, 1.38MB/s]
Downloading: 100%|████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████| 2.07k/2.07k [00:00<00:00, 1.15MB/s]
Downloading and preparing dataset imdb/plain_text (download: 80.23 MiB, generated: 127.06 MiB, post-processed: Unknown sizetotal: 207.28 MiB) to /net/vaosl01/opt/NFS/su0/huggingface/datasets/imdb/plain_text/1.0.0/76cdbd7249ea3548c928bbf304258dab44d09cd3638d9da8d42480d1d1be3743...
Downloading: 100%|████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████| 84.1M/84.1M [00:07<00:00, 11.1MB/s]
```

I checked the folder `$HF_HOME/datasets/downloads/extracted/<id>/aclImdb`. This folder is constantly growing in size. When I navigated to the train folder within, there was no file. However, the test folder seemed to be populating. The last time I checked it was 327M. I thought the Imdb dataset was smaller than that. My questions are:
1. Why is it still blocking? Is it still downloading?
2. I specified split as train, so why is the test folder being populated?
3. I read somewhere that after downloading, `nlp` converts the text files into some sort of `arrow` files, which will also take a while. Is this also happening here?

Thanks.
 
 I am also seeing a similar error when running the following:

```
import nlp
dataset = load_dataset('cola')
```
Error:
```
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/home/js11133/.conda/envs/jiant/lib/python3.8/site-packages/nlp/load.py", line 509, in load_dataset
    module_path = prepare_module(path, download_config=download_config, dataset=True)
  File "/home/js11133/.conda/envs/jiant/lib/python3.8/site-packages/nlp/load.py", line 248, in prepare_module
    local_path = cached_path(file_path, download_config=download_config)
  File "/home/js11133/.conda/envs/jiant/lib/python3.8/site-packages/nlp/utils/file_utils.py", line 191, in cached_path
    output_path = get_from_cache(
  File "/home/js11133/.conda/envs/jiant/lib/python3.8/site-packages/nlp/utils/file_utils.py", line 356, in get_from_cache
    raise ConnectionError("Couldn't reach {}".format(url))
ConnectionError: Couldn't reach https://s3.amazonaws.com/datasets.huggingface.co/nlp/datasets/cola/cola.py
```