I was finetuning bert-base-italian-cased on SQuAD-it dateset with the following arguments
args = TrainingArguments(
    f"test-squad_it",
    evaluation_strategy = "epoch",
    learning_rate=2e-5,
    per_device_train_batch_size=8,
    per_device_eval_batch_size=8,
    num_train_epochs=3,
    weight_decay=0.01,
    label_names = ["start_positions", "end_positions"]
)

trainer = Trainer(
    model,
    args,
    train_dataset=tokenized_datasets["train"],
    eval_dataset=tokenized_datasets["validation"],
    data_collator=data_collator,
    tokenizer=tokenizer,
)

Trying to do trainer.train() it suddenly throws this error:
KeyError                                  Traceback (most recent call last)
<ipython-input-168-4e078f57a6ea> in <module>()
      1 #We can now finetune our model by just calling the train method:
----> 2 trainer.train()

3 frames
/usr/local/lib/python3.7/dist-packages/transformers/trainer.py in train(self, resume_from_checkpoint, trial, **kwargs)
   1270                         tr_loss += self.training_step(model, inputs)
   1271                 else:
-> 1272                     tr_loss += self.training_step(model, inputs)
   1273                 self.current_flos += float(self.floating_point_ops(inputs))
   1274 

/usr/local/lib/python3.7/dist-packages/transformers/trainer.py in training_step(self, model, inputs)
   1732                 loss = self.compute_loss(model, inputs)
   1733         else:
-> 1734             loss = self.compute_loss(model, inputs)
   1735 
   1736         if self.args.n_gpu > 1:

/usr/local/lib/python3.7/dist-packages/transformers/trainer.py in compute_loss(self, model, inputs, return_outputs)
   1774         else:
   1775             # We don't use .loss here since the model may return tuples instead of ModelOutput.
-> 1776             loss = outputs["loss"] if isinstance(outputs, dict) else outputs[0]
   1777 
   1778         return (loss, outputs) if return_outputs else loss

/usr/local/lib/python3.7/dist-packages/transformers/file_utils.py in __getitem__(self, k)
   1736         if isinstance(k, str):
   1737             inner_dict = {k: v for (k, v) in self.items()}
-> 1738             return inner_dict[k]
   1739         else:
   1740             return self.to_tuple()[k]

KeyError: 'loss'

I’ve already had a look on similar questions about this same issue, but nothing seems to work and I’m really desperate. Any suggestions?
EDIT
My data are taken from SQuAD-it. I’ve created this dataDict:
DatasetDict({
    train: Dataset({
        features: ['answer_text', 'answer_start', 'title', 'context', 'question', 'answers', 'id'],
        num_rows: 48328
    })
    validation: Dataset({
        features: ['answer_text', 'answer_start', 'title', 'context', 'question', 'answers', 'id'],
        num_rows: 5831
    })
})

And then I’ve preprocessed the data as follows (from a tutorial with little adjustments):
def prepare_train_features(examples):
    # Tokenize our examples with truncation and padding, but keep the overflows using a stride. This results
    # in one example possible giving several features when a context is long, each of those features having a
    # context that overlaps a bit the context of the previous feature.
    tokenized_examples = tokenizer(
        examples["question" if pad_on_right else "context"],
        examples["context" if pad_on_right else "question"],
        truncation="only_second" if pad_on_right else "only_first",
        max_length=max_length,
        stride=doc_stride,
        return_overflowing_tokens=True,
        return_offsets_mapping=True,
        padding="max_length",
    )

    # Since one example might give us several features if it has a long context, we need a map from a feature to
    # its corresponding example. This key gives us just that.
    sample_mapping = tokenized_examples.pop("overflow_to_sample_mapping")
    # The offset mappings will give us a map from token to character position in the original context. This will
    # help us compute the start_positions and end_positions.
    offset_mapping = tokenized_examples.pop("offset_mapping")

    # Let's label those examples!
    tokenized_examples["start_positions"] = []
    tokenized_examples["end_positions"] = []

    for i, offsets in enumerate(offset_mapping):
        # We will label impossible answers with the index of the CLS token.
        input_ids = tokenized_examples["input_ids"][i]
        cls_index = input_ids.index(tokenizer.cls_token_id)

        # Grab the sequence corresponding to that example (to know what is the context and what is the question).
        sequence_ids = tokenized_examples.sequence_ids(i)

        # One example can give several spans, this is the index of the example containing this span of text.
        sample_index = sample_mapping[i]
        answers = examples["answers"][sample_index]
        # If no answers are given, set the cls_index as answer.
        if answers["answer_start"] == 0:
            tokenized_examples["start_positions"].append(cls_index)
            tokenized_examples["end_positions"].append(cls_index)
        else:
            # Start/end character index of the answer in the text.
            start_char = answers["answer_start"]
            end_char = start_char + len(answers["text"])

            # Start token index of the current span in the text.
            token_start_index = 0
            while sequence_ids[token_start_index] != (1 if pad_on_right else 0):
                token_start_index += 1

            # End token index of the current span in the text.
            token_end_index = len(input_ids) - 1
            while sequence_ids[token_end_index] != (1 if pad_on_right else 0):
                token_end_index -= 1

            # Detect if the answer is out of the span (in which case this feature is labeled with the CLS index).
            if not (offsets[token_start_index][0] <= start_char and offsets[token_end_index][1] >= end_char):
                tokenized_examples["start_positions"].append(cls_index)
                tokenized_examples["end_positions"].append(cls_index)
            else:
                # Otherwise move the token_start_index and token_end_index to the two ends of the answer.
                # Note: we could go after the last offset if the answer is the last word (edge case).
                while token_start_index < len(offsets) and offsets[token_start_index][0] <= start_char:
                    token_start_index += 1
                tokenized_examples["start_positions"].append(token_start_index - 1)
                while offsets[token_end_index][1] >= end_char:
                    token_end_index -= 1
                tokenized_examples["end_positions"].append(token_end_index + 1)

    return tokenized_examples

tokenized_datasets = train_dataset.map(prepare_train_features, batched=True, remove_columns=train_dataset["train"].column_names)

I’m quite new in coding and maybe I’m relying too much in other people’s work without understanding every detail.