[Question] BERT-style multiple choice formatting
Hello, I am wondering what the equivalent formatting of a dataset should be to allow for multiple-choice answering prediction, BERT-style. Previously, this was done by passing a list of `InputFeatures` to the dataloader instead of a list of `InputFeature`, where `InputFeatures` contained lists of length equal to the number of answer choices in the MCQ instead of single items. I'm a bit confused on what the output of my feature conversion function should be when using `dataset.map()` to ensure similar behavior.

Thanks!
Hi @sarahwie, can you details this a little more?

I'm not sure I understand what you refer to and what you mean when you say "Previously, this was done by passing a list of InputFeatures to the dataloader instead of a list of InputFeature" I think I've resolved it. For others' reference: to convert from using the [`MultipleChoiceDataset` class](https://github.com/huggingface/transformers/blob/a34a9896ac2a4a33ff9cd805c76eed914c8d8965/examples/multiple-choice/utils_multiple_choice.py#L82)/[`run_multiple_choice.py`](https://github.com/huggingface/transformers/blob/a34a9896ac2a4a33ff9cd805c76eed914c8d8965/examples/multiple-choice/run_multiple_choice.py) script in Huggingface Transformers, I've done the following for hellaswag:

1. converted the `convert_examples_to_features()` function to only take one input and return a dictionary rather than a list:
```
def convert_examples_to_features(example, tokenizer, max_length):

    choices_inputs = defaultdict(list)
    for ending_idx, ending in enumerate(example['endings']['ending']):
        text_a = example['ctx']
        text_b = ending

        inputs = tokenizer.encode_plus(
            text_a,
            text_b,
            add_special_tokens=True,
            max_length=max_length,
            pad_to_max_length=True,
            return_overflowing_tokens=True,
        )
        if "num_truncated_tokens" in inputs and inputs["num_truncated_tokens"] > 0:
            logger.info(
                "Attention! you are cropping tokens (swag task is ok). "
                "If you are training ARC and RACE and you are poping question + options,"
                "you need to try to use a bigger max seq length!"
            )

        for key in inputs:
            choices_inputs[key].append(inputs[key])
            
        choices_inputs['label'] = int(example['label'])

    return choices_inputs
```
2. apply this directly (instance-wise) to dataset, convert dataset to torch tensors. Dataset is then ready to be passed to `Trainer` instance.

```
dataset['train'] = dataset['train'].map(lambda x: convert_examples_to_features(x, tokenizer, max_length), batched=False)
columns = ['input_ids', 'token_type_ids', 'attention_mask', 'label']
dataset['train'].set_format(type='torch', columns=columns)
```