Sacrebleu unexpected behaviour/requirement for data format
Hi @BramVanroy!

Good question. This project relies on PyArrow (tables) to store data too big to fit in RAM. In the case of metrics, this means that the number of predictions and references has to match to form a table.

That's why your example throws an error even though it matches the schema:
```python
refs = [
    ['The dog bit the man.', 'It was not unexpected.', 'The man bit him first.'],
    ['The dog had bit the man.', 'No one was surprised.', 'The man had bitten the dog.'],
]  # len(refs) = 2

hyps = ['The dog bit the man.', "It wasn't surprising.", 'The man had just bitten him.'] # len(hyps) = 3
```

Instead, it should be:
```python
refs = [
    ['The dog bit the man.', 'The dog had bit the man.'],
    ['It was not unexpected.', 'No one was surprised.'],
    ['The man bit him first.', 'The man had bitten the dog.'], 
]  # len(refs) = 3

hyps = ['The dog bit the man.', "It wasn't surprising.", 'The man had just bitten him.'] # len(hyps) = 3
```

However, `sacreblue` works with the format that's described in your example, hence this part:
https://github.com/huggingface/datasets/blob/87c71b9c29a40958973004910f97e4892559dfed/metrics/sacrebleu/sacrebleu.py#L94-L99

Hope you get an idea! Thanks, that makes sense. It is a bit unfortunate because it may be confusing to users since the input format is suddenly different than what they may expect from the underlying library/metric. But it is understandable due to how `datasets` works!
## Describe the bug
When comparing with the original `sacrebleu` implementation, the `datasets` implementation does some strange things that I do not quite understand. This issue was triggered when I was trying to implement TER and found the datasets implementation of BLEU [here](https://github.com/huggingface/datasets/pull/3153).

In the below snippet, the original sacrebleu snippet works just fine whereas the datasets implementation throws an error.

## Steps to reproduce the bug
```python
import sacrebleu
import datasets

refs = [
    ['The dog bit the man.', 'It was not unexpected.', 'The man bit him first.'],
    ['The dog had bit the man.', 'No one was surprised.', 'The man had bitten the dog.'],
]

hyps = ['The dog bit the man.', "It wasn't surprising.", 'The man had just bitten him.']

expected_bleu = 48.530827

ds_bleu = datasets.load_metric("sacrebleu")

bleu_score_sb = sacrebleu.corpus_bleu(hyps, refs).score
print(bleu_score_sb, expected_bleu)
# works: 48.5308...
bleu_score_ds = ds_bleu.compute(predictions=hyps, references=refs)["score"]
print(bleu_score_ds, expected_bleu)
# ValueError: Predictions and/or references don't match the expected format.
```
This seems to be related to how datasets forces the features format here:

https://github.com/huggingface/datasets/blob/87c71b9c29a40958973004910f97e4892559dfed/metrics/sacrebleu/sacrebleu.py#L94-L99

and then manipulates the references during the compute stage here

https://github.com/huggingface/datasets/blob/87c71b9c29a40958973004910f97e4892559dfed/metrics/sacrebleu/sacrebleu.py#L119-L122

I do not quite understand why that is required since sacrebleu handles argument parsing quite well [by itself](https://github.com/mjpost/sacrebleu/blob/2787185dd0f8d224c72ee5a831d163c2ac711a47/sacrebleu/metrics/base.py#L229). 
## Actual results
Traceback (most recent call last):
  File "C:\Users\bramv\AppData\Roaming\JetBrains\PyCharm2020.3\scratches\scratch_23.py", line 23, in <module>
    bleu_score_ds = ds_bleu.compute(predictions=hyps, references=refs)["score"]
  File "C:\dev\python\datasets\src\datasets\metric.py", line 392, in compute
    self.add_batch(predictions=predictions, references=references)
  File "C:\dev\python\datasets\src\datasets\metric.py", line 439, in add_batch
    raise ValueError(
ValueError: Predictions and/or references don't match the expected format.
Expected format: {'predictions': Value(dtype='string', id='sequence'), 'references': Sequence(feature=Value(dtype='string', id='sequence'), length=-1, id='references')},
Input predictions: ['The dog bit the man.', "It wasn't surprising.", 'The man had just bitten him.'],
Input references: [['The dog bit the man.', 'It was not unexpected.', 'The man bit him first.'], ['The dog had bit the man.', 'No one was surprised.', 'The man had bitten the dog.']]

## Environment info
- `datasets` version: 1.14.1.dev0
- Platform: Windows-10-10.0.19041-SP0
- Python version: 3.9.2
- PyArrow version: 4.0.1

