JSONDecodeError with HuggingFace dataset viewer
Hi ! I think the issue comes from the dataset_infos.json file: it has the "flat" field twice.

Can you try deleting this file and regenerating it please ? Thanks! That fixed that, but now I am getting:
Server Error
Status code:   400
Exception:     KeyError
Message:       'feature'

I checked the dataset_infos.json and pubmed_neg.py script, I don't use 'feature' anywhere as a key. Is the dataset viewer expecting that I do? It seems that the `feature` key is missing from some feature type definition in your dataset_infos.json:
```json
			"tokens": {
				"dtype": "list",
				"id": null,
				"_type": "Sequence"
			},
			"tags": {
				"dtype": "list",
				"id": null,
				"_type": "Sequence"
			}
```
They should be
```json
			"tokens": {
				"dtype": "list",
				"id": null,
				"_type": "Sequence"
                                 "feature": {"dtype": "string", "id": null, "_type": "Value"}
			},
			"tags": {
				"dtype": "list",
				"id": null,
				"_type": "Sequence",
                                 "feature": {"num_classes": 5, "names": ["-", "S", "H", "N", "C"], "names_file": null, "id": null, "_type": "ClassLabel"}
			}
```

Note that you can generate the dataset_infos.json automatically to avoid mistakes:
```bash
datasets-cli test ./path/to/dataset --save_infos
```
## Dataset viewer issue for 'pubmed_neg'

**Link:** https://huggingface.co/datasets/IGESML/pubmed_neg

I am getting the error: 
Status code:   400
Exception:     JSONDecodeError
Message:       Expecting property name enclosed in double quotes: line 61 column 2 (char 1202)

I have checked all files - I am not using single quotes anywhere. Not sure what is causing this issue.

Am I the one who added this dataset ? Yes
