Issue: MemoryError when computing WER metric

Hi, I'm trying to follow the ASR example to try Wav2Vec. This is the code that I use for WER calculation:

```
wer = load_metric("wer")
print(wer.compute(predictions=result["predicted"], references=result["target"]))
```

However, I receive the following exception:

`Traceback (most recent call last):
  File "/home/diego/IpGlobal/wav2vec/test_wav2vec.py", line 51, in <module>
    print(wer.compute(predictions=result["predicted"], references=result["target"]))
  File "/home/diego/miniconda3/envs/wav2vec3.6/lib/python3.6/site-packages/datasets/metric.py", line 403, in compute
    output = self._compute(predictions=predictions, references=references, **kwargs)
  File "/home/diego/.cache/huggingface/modules/datasets_modules/metrics/wer/73b2d32b723b7fb8f204d785c00980ae4d937f12a65466f8fdf78706e2951281/wer.py", line 94, in _compute
    return wer(references, predictions)
  File "/home/diego/miniconda3/envs/wav2vec3.6/lib/python3.6/site-packages/jiwer/measures.py", line 81, in wer
    truth, hypothesis, truth_transform, hypothesis_transform, **kwargs
  File "/home/diego/miniconda3/envs/wav2vec3.6/lib/python3.6/site-packages/jiwer/measures.py", line 192, in compute_measures
    H, S, D, I = _get_operation_counts(truth, hypothesis)
  File "/home/diego/miniconda3/envs/wav2vec3.6/lib/python3.6/site-packages/jiwer/measures.py", line 273, in _get_operation_counts
    editops = Levenshtein.editops(source_string, destination_string)
MemoryError`

My system has more than 10GB of available RAM. Looking at the code, I think that it could be related to the way jiwer does the calculation, as it is pasting all the sentences in a single string before calling Levenshtein editops function.





Comments:
Hi ! Thanks for reporting.
We're indeed using `jiwer` to compute the WER.

Maybe instead of calling `jiwer.wer` once for all the preditions/references we can compute the WER iteratively to avoid memory issues ? I'm not too familial with `jiwer` but this must be possible.

Currently the code to compute the WER is defined here:

https://github.com/huggingface/nlp/blob/349ac4398a3bcae6356f14c5754483383a60e8a4/metrics/wer/wer.py#L93-L94

Hi,

I've just pushed a pull request that is related to this issue https://github.com/huggingface/datasets/pull/2169. It's not iterative, but it should avoid memory errors. It's based on the editdistance python library. An iterative implementation should be as easy as storing scores and words stepwise and dividing at the end.  

I see, this was solved by other thread. Ok, let me know if you want to switch the implementation for any reason :)

Thanks for diving into this anyway ^^'
As you said this actually got solved a few days ago

Someone created an issue https://github.com/jitsi/jiwer/issues/40 at jiwer which shows that this is still a problem in the current version. Would be curious to figure out how this can be fixed by jiwer... :) I assume that it runs of out memory because it's trying to compute the WER over (too many) test samples?

Hi !

It's computed iteratively so not sure what could go wrong

https://github.com/huggingface/datasets/blob/8afd0ba8c27800a55ea69d9fcd702dc97d9c16d8/metrics/wer/wer.py#L100-L106

@NiklasHoltmeyer what version of `datasets` are you running ?


One possible explanation might be that it is the user who is passing all the sentences in a single element to `wer.compute`?

As current implementation iterates over the elements of `predictions` and `references`, this can be problematic if `predictions` and `references` contain a single huge element each. 

This could be the case, for example, with a single string with all sentences:
```python
result["predicted"] = "One sentence. Other sentence."
```
or with a __double__ nested list of sentence lists
```python
result["predicted"] = [[ ["One sentence."], ["Other sentence"] ]]
```

The user should check the dimensions of the data structure passed to `predictions` and `references`.

Hi all,

in my case I was using and older version of datasets and, as @albertvillanova points out, passing the full list of sentences for the metric calculation. The problem was in the way jiwer implements WER, as it tries to compute WER for the full list at once instead of doing it element-wise. I think that with the latest implementation of datasets, or by using the alternative WER function that I've contributed on this [pull request](https://github.com/huggingface/datasets/pull/2169) there shouldn't be memory errors.

@lhoestq i was using Datasets==1.5.0 with 1.6.1 it worked (atleast the first run) but 1.5.0 is not compatible with my preprocessing. i cant save my dataset to a parquet file while using the latest datasets version

-> 
```
  File "../preprocess_dataset.py", line 132, in <module>
    pq.write_table(train_dataset.data, f'{resampled_data_dir}/{data_args.dataset_config_name}.train.parquet')
  File "/usr/local/lib/python3.8/dist-packages/pyarrow/parquet.py", line 1674, in write_table
    writer.write_table(table, row_group_size=row_group_size)
  File "/usr/local/lib/python3.8/dist-packages/pyarrow/parquet.py", line 588, in write_table
    self.writer.write_table(table, row_group_size=row_group_size)
TypeError: Argument 'table' has incorrect type (expected pyarrow.lib.Table, got ConcatenationTable)
``` 

if i do 
```
import pyarrow.parquet as pq
...
...
pq.write_table(train_dataset.data, 'train.parquet')
pq.write_table(eval_dataset.data, 'eval.parquet')
```

while using 1.6.1. and its working with 1.5.0


Hi ! You can pass dataset.data.table instead of dataset.data to pq.write_table

This seems to be working so far! Thanks!