Fix manual download instructions
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.