Hello everyone. I already have post a question about fine-tuning bert-base-italian-cased on SQuAD-it dateset. Waiting for an answer I tried another solution, following the Question Answerinf tutorial on SQuAS 2.0 in the transformers docs on HuggingFace.
My data are taken from SQuAD-it. I followed this way:
import json
from pathlib import Path

def read_dataset(path):
    path = Path(path)
    with open(path, 'rb') as f:
        squad_dict = json.load(f)

    contexts = []
    questions = []
    answers = []
    for group in squad_dict['data']:
        for passage in group['paragraphs']:
            context = passage['context']
            for qa in passage['qas']:
                question = qa['question']
                for answer in qa['answers']:
                    contexts.append(context)
                    questions.append(question)
                    answers.append(answer)

    return contexts, questions, answers

train_contexts, train_questions, train_answers = read_dataset('SQuAD_it-train.json')

val_contexts = []
val_questions = []
val_answers = []



while len(val_answers) != 5831:
  value = train_contexts.pop()
  val_contexts.append(value)
  value = train_questions.pop()
  val_questions.append(value)
  value = train_answers.pop()
  val_answers.append(value)

def add_end_idx(answers, contexts):
    for answer, context in zip(answers, contexts):
        gold_text = answer['text']
        start_idx = answer['answer_start']
        end_idx = start_idx + len(gold_text)

        # sometimes squad answers are off by a character or two – fix this
#        if context[start_idx:end_idx] == gold_text:
#            answer['answer_end'] = end_idx
        if context[start_idx-1:end_idx-1] == gold_text:
            answer['answer_start'] = start_idx - 1
            answer['answer_end'] = end_idx - 1     # When the gold label is off by one character
        elif context[start_idx-2:end_idx-2] == gold_text:
            answer['answer_start'] = start_idx - 2
            answer['answer_end'] = end_idx - 2     # When the gold label is off by two characters
        elif context[start_idx-1:end_idx-2] == gold_text:
            answer['answer_start'] = start_idx - 1
            answer['answer_end'] = end_idx - 2
        elif context[start_idx-2:end_idx-1] == gold_text:
            answer['answer_start'] = start_idx - 2
            answer['answer_end'] = end_idx - 1
        elif context[start_idx-3:end_idx-3] == gold_text:
            answer['answer_start'] = start_idx - 3
            answer['answer_end'] = end_idx - 3
        elif context[start_idx-2:end_idx-3] == gold_text:
            answer['answer_start'] = start_idx - 2
            answer['answer_end'] = end_idx - 3
        elif context[start_idx-3:end_idx-2] == gold_text:
            answer['answer_start'] = start_idx - 3
            answer['answer_end'] = end_idx - 2
        else:
          answer['answer_end'] = end_idx
        if answer['answer_start'] < 0:
            answer['answer_start'] =+ 1
            answer['answer_end'] =+ 1

add_end_idx(train_answers, train_contexts)
add_end_idx(val_answers, val_contexts)

from transformers import DistilBertTokenizerFast
tokenizer = DistilBertTokenizerFast.from_pretrained('dbmdz/bert-base-italian-cased')

train_encodings = tokenizer(train_contexts, train_questions, truncation=True, padding=True)
val_encodings = tokenizer(val_contexts, val_questions, truncation=True, padding=True)

from transformers import AutoModel,
model_name = "dbmdz/bert-base-italian-cased"
model = AutoModel.from_pretrained(model_name)

def add_token_positions(encodings, answers):
    start_positions = []
    end_positions = []
    for i in range(len(answers)):
        start_positions.append(encodings.char_to_token(i, answers[i]['answer_start']))
        end_positions.append(encodings.char_to_token(i, answers[i]['answer_end'] - 1))

        # if start position is None, the answer passage has been truncated
        if start_positions[-1] is None:
            start_positions[-1] = tokenizer.model_max_length
        if end_positions[-1] is None:
            end_positions[-1] = tokenizer.model_max_length

    encodings.update({'start_positions': start_positions, 'end_positions': end_positions})

add_token_positions(train_encodings, train_answers)
add_token_positions(val_encodings, val_answers)

Then I created the datasets:
import torch

class SquadDataset(torch.utils.data.Dataset):
    def __init__(self, encodings):
        self.encodings = encodings

    def __getitem__(self, idx):
        return {key: torch.tensor(val[idx]) for key, val in self.encodings.items()}

    def __len__(self):
        return len(self.encodings.input_ids)

train_dataset = SquadDataset(train_encodings)
val_dataset = SquadDataset(val_encodings)

And finally I tried to train:
from transformers import TrainingArguments, Trainer

training_args = TrainingArguments(
    output_dir='./results',          # output directory
    num_train_epochs=3,              # total number of training epochs
    per_device_train_batch_size=16,  # batch size per device during training
    per_device_eval_batch_size=64,   # batch size for evaluation
    warmup_steps=500,                # number of warmup steps for learning rate scheduler
    weight_decay=0.01,               # strength of weight decay
    logging_dir='./logs',            # directory for storing logs
    logging_steps=10,
    label_names = ["start_positions", "end_positions"]
)

trainer = Trainer(
    model=model,                         # the instantiated 🤗 Transformers model to be trained
    args=training_args,                  # training arguments, defined above
    train_dataset=train_dataset,         # training dataset
    eval_dataset=val_dataset             # evaluation dataset
)

trainer.train()

But it raises me this error:
---------------------------------------------------------------------------
TypeError                                 Traceback (most recent call last)
<ipython-input-171-8794092ae722> in <module>()
     20 )
     21 
---> 22 trainer.train()

3 frames
/usr/local/lib/python3.7/dist-packages/torch/nn/modules/module.py in _call_impl(self, *input, **kwargs)
    887             result = self._slow_forward(*input, **kwargs)
    888         else:
--> 889             result = self.forward(*input, **kwargs)
    890         for hook in itertools.chain(
    891                 _global_forward_hooks.values(),

TypeError: forward() got an unexpected keyword argument 'start_positions'

I’ve seen this has already an issue but in none topic I’ve found a solution. Thank you in advance.