Seq2Seq Metrics QOL: Bleu, Rouge
Hi ! Thanks for letting us know your experience :) 
We should at least improve the error messages indeed So what is the right way to add a batch to compute BLEU? prediction = [['Hey', 'how', 'are', 'you', '?']]  
reference=[['Hey', 'how', 'are', 'you', '?']]
bleu.compute(predictions=prediction,references=reference)

also tried this kind of things lol
I definitely need help too Hi !

As described in the documentation for `bleu`:
```
Args:
    predictions: list of translations to score.
        Each translation should be tokenized into a list of tokens.
    references: list of lists of references for each translation.
        Each reference should be tokenized into a list of tokens.
```

Therefore you can use this metric this way:
```python
from datasets import load_metric

predictions = [
    ["hello", "there", "general", "kenobi"],                             # tokenized prediction of the first sample
    ["foo", "bar", "foobar"]                                             # tokenized prediction of the second sample
]
references = [
    [["hello", "there", "general", "kenobi"], ["hello", "there", "!"]],  # tokenized references for the first sample (2 references)
    [["foo", "bar", "foobar"]]                                           # tokenized references for the second sample (1 reference)
]

bleu = load_metric("bleu")
bleu.compute(predictions=predictions, references=references)
# Or you can also add batches before calling compute()
# bleu.add_batch(predictions=predictions, references=references)
# bleu.compute()
```

Hope this helps :)
Putting all my QOL issues here, idt I will have time to propose fixes, but I didn't want these to be lost, in case they are useful. I tried using `rouge` and `bleu` for the first time and wrote down everything I didn't immediately understand:

+ Bleu expects tokenization, can I just kwarg it like sacrebleu?
+ different signatures, means that I would have had to add a lot of conditionals + pre and post processing: if I were going to replace the `calculate_rouge` and `calculate_bleu` functions here: https://github.com/huggingface/transformers/blob/master/examples/seq2seq/utils.py#L61


#### What I tried


Rouge experience:
```python

rouge = load_metric('rouge')
rouge.add_batch(['hi im sam'], ['im daniel']) # fails
rouge.add_batch(predictions=['hi im sam'], references=['im daniel']) # works
rouge.compute() # huge messy output, but reasonable. Not worth integrating b/c don't want to rewrite all the postprocessing.
```

BLEU experience:
```python
bleu = load_metric('bleu')
bleu.add_batch(predictions=['hi im sam'], references=['im daniel'])
bleu.add_batch(predictions=[['hi im sam']], references=[['im daniel']])

bleu.add_batch(predictions=[['hi im sam']], references=[['im daniel']])
```
All of these raise `ValueError: Got a string but expected a list instead: 'im daniel'`

#### Doc Typo
This says `dataset=load_metric(...)` which seems wrong, will cause `NameError`

![image](https://user-images.githubusercontent.com/6045025/98004483-ff0d0580-1dbd-11eb-9f35-6f35904611bb.png)

cc @lhoestq, feel free to ignore.