Loading an external dataset in a format similar to conll2003
I'm trying to load a custom dataset in a Dataset object, it's similar to conll2003 but with 2 columns only (word entity), I used the following script:

features = datasets.Features(
            {"tokens": datasets.Sequence(datasets.Value("string")),
        "ner_tags": datasets.Sequence(
            datasets.features.ClassLabel(
                names=["B-PER", .... etc.]))}
        )


from datasets import Dataset

INPUT_COLUMNS = "tokens ner_tags".split(" ")

def read_conll(file):
    #all_labels = []
    example = {col: [] for col in INPUT_COLUMNS}
    idx = 0
    with open(file) as f:
        for line in f:
          if line:
            if line.startswith("-DOCSTART-") and example["tokens"] != []:
                print(idx, example)
                yield idx, example
                idx += 1
                example = {col: [] for col in INPUT_COLUMNS}
            elif line == "\n" or (line.startswith("-DOCSTART-") and example["tokens"] == []):
              continue
            else:
                row_cols = line.split(" ")
                for i, col in enumerate(example):
                    example[col] = row_cols[i].rstrip()

dset = Dataset.from_generator(read_conll, gen_kwargs={"file": "/content/new_train.txt"}, features = features)   

The following error happened:
[/usr/local/lib/python3.7/dist-packages/datasets/utils/py_utils.py](https://localhost:8080/#) in <genexpr>(.0)
    285     for key in unique_values(itertools.chain(*dicts)):  # set merge all keys
    286         # Will raise KeyError if the dict don't have the same keys
--> 287         yield key, tuple(d[key] for d in dicts)
    288 
    

TypeError: tuple indices must be integers or slices, not str

What does this mean and what should I modify?