Fix manual download instructions
{'login': 'patrickvonplaten', 'id': 23423619, 'node_id': 'MDQ6VXNlcjIzNDIzNjE5', 'avatar_url': 'https://avatars.githubusercontent.com/u/23423619?v=4', 'gravatar_id': '', 'url': 'https://api.github.com/users/patrickvonplaten', 'html_url': 'https://github.com/patrickvonplaten', 'followers_url': 'https://api.github.com/users/patrickvonplaten/followers', 'following_url': 'https://api.github.com/users/patrickvonplaten/following{/other_user}', 'gists_url': 'https://api.github.com/users/patrickvonplaten/gists{/gist_id}', 'starred_url': 'https://api.github.com/users/patrickvonplaten/starred{/owner}{/repo}', 'subscriptions_url': 'https://api.github.com/users/patrickvonplaten/subscriptions', 'organizations_url': 'https://api.github.com/users/patrickvonplaten/orgs', 'repos_url': 'https://api.github.com/users/patrickvonplaten/repos', 'events_url': 'https://api.github.com/users/patrickvonplaten/events{/privacy}', 'received_events_url': 'https://api.github.com/users/patrickvonplaten/received_events', 'type': 'User', 'site_admin': False}
[]
[]
Verified that this works, thanks! But I get
```python
ConnectionError: Couldn't reach https://s3.amazonaws.com/datasets.huggingface.co/nlp/datasets/./datasets/wmt16/wmt16.py
```
When I try from jupyter on brutasse or my mac. (the jupyter server is run from transformers).


Both machines can run
```bash
aws s3 ls s3://datasets.huggingface.co/nlp/datasets/wmt16/
```
but it seems one must be in the nlp directory to run the command?

(I ran `pip install -e . ` on this branch in both situations.)


 `https://s3.amazonaws.com/datasets.huggingface.co/nlp/datasets/./datasets/wmt16/wmt16.py` looks very weird.

(Also, S3 is not a file-system, it's a flat key-value store) Good to merge I think @lhoestq  > But I get
> 
> ```python
> ConnectionError: Couldn't reach https://s3.amazonaws.com/datasets.huggingface.co/nlp/datasets/./datasets/wmt16/wmt16.py
> ```
> 
> When I try from jupyter on brutasse or my mac. (the jupyter server is run from transformers).
> 
> Both machines can run
> 
> ```shell
> aws s3 ls s3://datasets.huggingface.co/nlp/datasets/wmt16/
> ```
> 
> but it seems one must be in the nlp directory to run the command?
> 
> (I ran `pip install -e . ` on this branch in both situations.)

As soon as it is on master, the dataset script wmt16.py will be synced on S3 and you'll be able to do `load_dataset("wmt16")`
This PR replaces the static `DatasetBulider` variable `MANUAL_DOWNLOAD_INSTRUCTIONS` by a property function `manual_download_instructions()`. 

Some datasets like XTREME and all WMT need the manual data dir only for a small fraction of the possible configs.

After some brainstorming with @mariamabarham and @lhoestq, we came to the conclusion that having a property function `manual_download_instructions()` gives us more flexibility to decide on a per config basis in the dataset builder if manual download instructions are needed.

Also this PR should unblock solves a bug with `wmt16 - ro-en` 
@sshleifer from this branch you should be able to succesfully run

```python 
import nlp 
ds = nlp.load_dataset('./datasets/wmt16', 'ro-en')
```

and once this PR is merged S3 should be synched so that 

```python
import nlp
ds = nlp.load_dataset("wmt16", "ro-en")
```

works as well.

**Important**: Since `MANUAL_DOWNLOAD_INSTRUCTIONS` was not really exposed to the user, this PR should not be a problem regarding backward compatibility.
{'url': 'https://api.github.com/repos/huggingface/datasets/issues/284/reactions', 'total_count': 0, '+1': 0, '-1': 0, 'laugh': 0, 'hooray': 0, 'confused': 0, 'heart': 0, 'rocket': 0, 'eyes': 0}
{'url': 'https://api.github.com/repos/huggingface/datasets/pulls/284', 'html_url': 'https://github.com/huggingface/datasets/pull/284', 'diff_url': 'https://github.com/huggingface/datasets/pull/284.diff', 'patch_url': 'https://github.com/huggingface/datasets/pull/284.patch', 'merged_at': '2020-06-19T08:24:19Z'}