custom JSON data breaks run_seq2seq.py
## Environment info
- `transformers` version: 4.4.0.dev0
- Platform: Linux-4.19.0-11-amd64-x86_64-with-debian-10.6
- Python version: 3.7.3
- PyTorch version (GPU?): 1.7.1 (True)
- Tensorflow version (GPU?): not installed (NA)
- Using GPU in script?: <fill in>
- Using distributed or parallel set-up in script?: <fill in>

### Who can help
 @sgugger, @patil-suraj

## Information

I'm trying to use mt5 model (but same happens with the example t5-small)

The problem arises when using:
* [ x ] the official example scripts: (give details below)

## To reproduce

When I try to run example/seq2se1/run_seq2seq.py with my own data files, the following error occurs:

```Downloading and preparing dataset json/default-f1b6ef8723ed4d49 (download: Unknown size, generated: Unknown size, post-processed: Unknown size, total: Unknown size) to /home/xxxxxx/.cache/huggingface/datasets/json/default-f1b6ef8723ed4d49/0.0.0/70d89ed4db1394f028c651589fcab6d6b28dddcabbe39d3b21b4d41f9a708514...
Traceback (most recent call last):
  File "/home/xxxxx/.cache/huggingface/modules/datasets_modules/datasets/json/70d89ed4db1394f028c651589fcab6d6b28dddcabbe39d3b21b4d41f9a708514/json.py", line 82, in _generate_tables
    parse_options=self.config.pa_parse_options,
  File "pyarrow/_json.pyx", line 247, in pyarrow._json.read_json
  File "pyarrow/error.pxi", line 122, in pyarrow.lib.pyarrow_internal_check_status
  File "pyarrow/error.pxi", line 84, in pyarrow.lib.check_status
pyarrow.lib.ArrowInvalid: JSON parse error: Column() changed from object to array in row 0

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
  File "examples/seq2seq/run_seq2seq.py", line 537, in <module>
    main()
  File "examples/seq2seq/run_seq2seq.py", line 287, in main
    datasets = load_dataset(extension, data_files=data_files)
  File "/home/xxxxx/.local/lib/python3.7/site-packages/datasets/load.py", line 612, in load_dataset
    ignore_verifications=ignore_verifications,
  File "/home/xxxxx/.local/lib/python3.7/site-packages/datasets/builder.py", line 527, in download_and_prepare
    dl_manager=dl_manager, verify_infos=verify_infos, **download_and_prepare_kwargs
  File "/home/xxxxx/.local/lib/python3.7/site-packages/datasets/builder.py", line 604, in _download_and_prepare
    self._prepare_split(split_generator, **prepare_split_kwargs)
  File "/home/xxxx/.local/lib/python3.7/site-packages/datasets/builder.py", line 959, in _prepare_split
    for key, table in utils.tqdm(generator, unit=" tables", leave=False, disable=not_verbose):
  File "/home/xxxx/.local/lib/python3.7/site-packages/tqdm/std.py", line 1166, in __iter__
    for obj in iterable:
  File "/home/xxxxxxxx/.cache/huggingface/modules/datasets_modules/datasets/json/70d89ed4db1394f028c651589fcab6d6b28dddcabbe39d3b21b4d41f9a708514/json.py", line 88, in _generate_tables
    f"Not able to read records in the JSON file at {file}. "
AttributeError: 'list' object has no attribute 'keys'
```

I'm using the following json format:
```
[
  {
  "title": "summarised text",
   "body": "text to be summarized"
  },
{
   "title": "summarised text",
   "body": "text to be summarized"
  }
]
```

I pass the --text_column body --summary_column title to the script and I can't understand why it's breaking.


Hi @varna9000 

The script expects jsonline format where each line is a `json` object. for ex:
```
{"title": "summarised text","body": "text to be summarized"}
{"title": "summarised text","body": "text to be summarized"}
```

You can find more info in the `datasets` doc here https://huggingface.co/docs/datasets/loading_datasets.html#json-files<|||||>Thank you!  :)<|||||>Hi, I somehow encountered the same problem. Can I ask you how did you solve it?<|||||>> Hi, I somehow encountered the same problem. Can I ask you how did you solve it?

Encountered too.<|||||>@judy-jii @hihihihiwsf it has been answered. You have to pass jsonlines format, not json.<|||||>For anyone else who gets here from google, make sure your JSON lines are not wrapped in quote marks or something similar. Each line should be a valid JSON object, not a string. 