Make tokenizer.pad() also pad `labels`
# 🚀 Feature request

Make tokenizer.pad() also pad `labels`

## Motivation

I tried to use this:
https://github.com/huggingface/transformers/blob/8065fea87007fbf7542fc060ff8ddd0b5df567da/src/transformers/data/data_collator.py#L69 

But since labels is not padded, the result cannot turn into a tensor. `ValueError: Unable to create tensor, you should probably activate truncation and/or padding with 'padding=True' 'truncation=True' to have batched tensors with the same length.`

It currently pads `input_ids, attention_mask, token_type_ids, special_tokens_mask`
It seems logical to me that `tokenizer.pad()` should also pad `'labels'`. 

## Your contribution

I have already created a PR #8116. It solves the problem above.