Fix manual download instructions
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.