Keys should be unique error on code_search_net

## Describe the bug
Loading `code_search_net` seems not possible at the moment.

## Steps to reproduce the bug
```python
>>> load_dataset('code_search_net')
Downloading: 8.50kB [00:00, 3.09MB/s]                                                                                                                                           
Downloading: 19.1kB [00:00, 10.1MB/s]                                                                                                                                           
No config specified, defaulting to: code_search_net/all
Downloading and preparing dataset code_search_net/all (download: 4.77 GiB, generated: 5.99 GiB, post-processed: Unknown size, total: 10.76 GiB) to /Users/thomwolf/.cache/huggingface/datasets/code_search_net/all/1.0.0/b3e8278faf5d67da1d06981efbeac3b76a2900693bd2239bbca7a4a3b0d6e52a...
Traceback (most recent call last):         
  File "/Users/thomwolf/Documents/GitHub/datasets/src/datasets/builder.py", line 1067, in _prepare_split
    writer.write(example, key)
  File "/Users/thomwolf/Documents/GitHub/datasets/src/datasets/arrow_writer.py", line 343, in write
    self.check_duplicate_keys()
  File "/Users/thomwolf/Documents/GitHub/datasets/src/datasets/arrow_writer.py", line 354, in check_duplicate_keys
    raise DuplicatedKeysError(key)
datasets.keyhash.DuplicatedKeysError: FAILURE TO GENERATE DATASET !
Found duplicate Key: 48
Keys should be unique and deterministic in nature
```

## Environment info
- `datasets` version: 1.8.1.dev0
- Platform: macOS-10.15.7-x86_64-i386-64bit
- Python version: 3.8.5
- PyArrow version: 2.0.0


Labels: name: bug

Two questions:
- with `datasets-cli env` we don't have any information on the dataset script version used. Should we give access to this somehow? Either as a note in the Error message or as an argument with the name of the dataset to `datasets-cli env`?
- I don't really understand why the id is duplicated in the code of `code_search_net`, how can I debug this actually?

Thanks for reporting. There was indeed an issue with the keys. The key was the addition of the file id and row id, which resulted in collisions. I just opened a PR to fix this at https://github.com/huggingface/datasets/pull/2555

To help users debug this kind of errors we could try to show a message like this
```python
DuplicateKeysError: both 42th and 1337th examples have the same keys `48`.
Please fix the dataset script at <path/to/the/dataset/script>
```

This way users who what to look for if they want to debug this issue. I opened an issue to track this: https://github.com/huggingface/datasets/issues/2556

and are we sure there are not a lot of datasets which are now broken with this change?

Thanks to the dummy data, we know for sure that most of them work as expected.
`code_search_net` wasn't caught because the dummy data only have one dummy data file while the dataset script can actually load several of them using `os.listdir`. Let me take a look at all the other datasets that use `os.listdir` to see if the keys are alright

I found one issue on `fever` (PR here: https://github.com/huggingface/datasets/pull/2557)
All the other ones seem fine :)

Hi! Got same error when loading other dataset:
```python3
load_dataset('wikicorpus', 'raw_en')
```

tb:
```pytb
---------------------------------------------------------------------------
DuplicatedKeysError                       Traceback (most recent call last)
/opt/conda/lib/python3.8/site-packages/datasets/builder.py in _prepare_split(self, split_generator)
   1109                     example = self.info.features.encode_example(record)
-> 1110                     writer.write(example, key)
   1111             finally:

/opt/conda/lib/python3.8/site-packages/datasets/arrow_writer.py in write(self, example, key, writer_batch_size)
    341             if self._check_duplicates:
--> 342                 self.check_duplicate_keys()
    343                 # Re-intializing to empty list for next batch

/opt/conda/lib/python3.8/site-packages/datasets/arrow_writer.py in check_duplicate_keys(self)
    352             if hash in tmp_record:
--> 353                 raise DuplicatedKeysError(key)
    354             else:

DuplicatedKeysError: FAILURE TO GENERATE DATASET !
Found duplicate Key: 519
Keys should be unique and deterministic in nature
```

Version: datasets==1.11.0

Fixed by #2555.

The wikicorpus issue has been fixed by https://github.com/huggingface/datasets/pull/2844

We'll do a new release of `datasets` soon :)