I need to read the custom dataset in conll format
Hi! We don't currently have a builder for parsing custom `conll` datasets, but I guess we could add one as a packaged module (similarly to what [TFDS](https://github.com/tensorflow/datasets/blob/master/tensorflow_datasets/core/dataset_builders/conll/conll_dataset_builder.py) did). @lhoestq @albertvillanova WDYT?

In the meantime, you can use `Dataset.from_generator` to create a dataset as follows:
```python
from datasets import Dataset

# 2009 version
INPUT_COLUMNS = "ID FORM LEMMA PLEMMA POS PPOS FEAT PFEAT HEAD PHEAD DEPREL PDEPREL".split()

def read_conll(file):
    example = {col: [] for col in INPUT_COLUMNS}
    idx = 0
    with open(file) as f:
        for line in f:
            if line.startswith("-DOCSTART-") or line == "\n" or not line:
                if example[next(iter(example))]:
                    yield idx, example
                idx += 1
                example = {col: [] for col in INPUT_COLUMNS}
            else:
                row_cols = line.split()
                for i, col in enumerate(example):
                    example[col] = row_cols[i].rstrip()

# (optional) pass custom features with `features=Features(...)`
dset = Dataset.from_generator(read_conll, gen_kwargs={"file": "path/to/conll/file"})   
```  I think we could add a dedicated builder if you think this format is general enough. 


> I think we could add a dedicated builder if you think this format is general enough.

I think its functions are incomplete. It should have to_ Conll and from_ There are two methods of conll.
I need to read the custom dataset in conll format
