Issue: Datasets.py function load_dataset does not match squad dataset

Comment:
## I have fixed it, @lhoestq 
### the first section change as you said and add ["id"]
```python
def process_squad(examples):
    """
    Process a dataset in the squad format with columns "title" and "paragraphs"
    to return the dataset with columns "context", "question" and "answers".
    """
    # print(examples)
    out = {"context": [], "question": [], "answers":[],"id":[]} 
    for paragraphs in examples["paragraphs"]: 
        for paragraph in paragraphs: 
            for qa in paragraph["qas"]: 
                answers = [{"answer_start": answer["answer_start"], "text": answer["text"].strip()} for answer in qa["answers"]] 
                out["context"].append(paragraph["context"].strip()) 
                out["question"].append(qa["question"].strip()) 
                out["answers"].append(answers) 
                out["id"].append(qa["id"]) 
    return out
column_names = datasets["train"].column_names if training_args.do_train else datasets["validation"].column_names
# print(datasets["train"].column_names)
if set(column_names) == {"title", "paragraphs"}:
    datasets = datasets.map(process_squad, batched=True, remove_columns=column_names)
# Preprocessing the datasets.
# Preprocessing is slighlty different for training and evaluation.
if training_args.do_train:
    column_names = datasets["train"].column_names
else:
    column_names = datasets["validation"].column_names
# print(column_names)
question_column_name = "question" if "question" in column_names else column_names[0]
context_column_name = "context" if "context" in column_names else column_names[1]
answer_column_name = "answers" if "answers" in column_names else column_names[2]
```
### the second section
```python
def prepare_train_features(examples):
    # Tokenize our examples with truncation and maybe 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_column_name if pad_on_right else context_column_name],
        examples[context_column_name if pad_on_right else question_column_name],
        truncation="only_second" if pad_on_right else "only_first",
        max_length=data_args.max_seq_length,
        stride=data_args.doc_stride,
        return_overflowing_tokens=True,
        return_offsets_mapping=True,
        padding="max_length" if data_args.pad_to_max_length else False,
    )

    # 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[answer_column_name][sample_index]
        # print(examples,answers,offset_mapping,tokenized_examples)
        # If no answers are given, set the cls_index as answer.
        if len(answers) == 0:#len(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[0]["answer_start"]
            end_char = start_char + len(answers[0]["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
```