[WebDataset] KeyError with user-defined `Features` when a field is missing in an example
@lhoestq How difficult of fix is this? It shouldn't be difficult, I think it's just a matter of adding the missing fields from `self.config.features` in `example` here: before it iterates on image_field_names and audio_field_names. A missing field should have a value set to None

https://github.com/huggingface/datasets/blob/768cb35ede5a6c35fa7545aa3671f3e321c96440/src/datasets/packaged_modules/webdataset/webdataset.py#L113-L116 @lhoestq So like this then?

```    
def _generate_examples(self, tar_paths, tar_iterators):
        image_field_names = [
            field_name for field_name, feature in self.info.features.items() if isinstance(feature, datasets.Image)
        ]
        audio_field_names = [
            field_name for field_name, feature in self.info.features.items() if isinstance(feature, datasets.Audio)
        ]
	
        all_field_names = list(self.config.features.keys())
    
        for tar_idx, (tar_path, tar_iterator) in enumerate(zip(tar_paths, tar_iterators)):
            for example_idx, example in enumerate(self._get_pipeline_from_tar(tar_path, tar_iterator)):
                for field_name in all_field_names:
                    if field_name not in example:
                        if field_name in self.config.features:
                            example[field_name] = self.config.features[field_name]
                        else:
                            example[field_name] = None
    
                # Process image and audio fields
                for field_name in image_field_names + audio_field_names:
                    if example[field_name] is not None:
                        example[field_name] = {"path": example["__key__"] + "." + field_name, "bytes": example[field_name]}
    
                yield f"{tar_idx}_{example_idx}", example
```

Or should we avoid trying add the missing values and just set them to None?

```
            for field_name in all_field_names:
                if field_name not in example:
                    example[field_name] = None
``` Yup this is the solution !

```python
            for field_name in all_field_names:
                if field_name not in example:
                    example[field_name] = None
``` @lhoestq Awesome, thanks! I made a PR with the fixes
reported at https://huggingface.co/datasets/ProGamerGov/synthetic-dataset-1m-dalle3-high-quality-captions/discussions/1

```
File "/src/services/worker/.venv/lib/python3.9/site-packages/datasets/packaged_modules/webdataset/webdataset.py", line 109, in _generate_examples
    example[field_name] = {"path": example["__key__"] + "." + field_name, "bytes": example[field_name]}
```