Nested sequences with dicts
Oh yes, this is a backward compatibility feature with tensorflow_dataset in which a `Sequence` or `dict` is converted in a `dict` of `lists`, unfortunately it is not very intuitive, see here: https://github.com/huggingface/nlp/blob/master/src/nlp/features.py#L409

To avoid this behavior, you can just define the list in the feature with a simple list or a tuple (which is also simpler to write).
In your case, the features could be as follow:
``` python
...
features=nlp.Features({
    "title": nlp.Value("string"),
    "vertexSet": [[{
        "name": nlp.Value("string"),
        "sent_id": nlp.Value("int32"),
        "pos": nlp.features.Sequence(nlp.Value("int32")),
        "type": nlp.Value("string"),
    }]],
    ...
    }),
...
```
Am pretty much finished [adding a dataset](https://github.com/ghomasHudson/nlp/blob/DocRED/datasets/docred/docred.py) for [DocRED](https://github.com/thunlp/DocRED), but am getting an error when trying to add a nested `nlp.features.sequence(nlp.features.sequence({key:value,...}))`. 

The original data is in this format:
```python
{
  'title': "Title of wiki page",
  'vertexSet': [
                  [
                    { 'name': "mention_name", 
                      'sent_id': "mention in which sentence", 
                      'pos': ["postion of mention in a sentence"], 
                      'type': "NER_type"},
                    {another mention}
                  ], 
                  [another entity]
                ]
    ...
}
```
So to represent this I've attempted to write:
```
...
features=nlp.Features({
    "title": nlp.Value("string"),
    "vertexSet": nlp.features.Sequence(nlp.features.Sequence({
        "name": nlp.Value("string"),
        "sent_id": nlp.Value("int32"),
        "pos": nlp.features.Sequence(nlp.Value("int32")),
        "type": nlp.Value("string"),
    })),
    ...
    }),
...
```
This is giving me the error:
```
pyarrow.lib.ArrowTypeError: Could not convert [{'pos': [[0,2], [2,4], [3,5]], "type": ["ORG", "ORG", "ORG"], "name": ["Lark Force", "Lark Force", "Lark Force", "sent_id": [0, 3, 4]}..... with type list: was not a dict, tuple, or recognized null value for conversion to struct type
```
Do we expect the pyarrow stuff to break when doing this deeper nesting? I've checked that it still works when you do `nlp.features.Sequence(nlp.features.Sequence(nlp.Value("string"))` or `nlp.features.Sequence({key:value,...})` just not nested sequences with a dict.

If it's not possible, I can always convert it to a shallower structure. I'd rather not change the DocRED authors' structure if I don't have to though.