Add align_labels_with_mapping function
This PR adds a helper function to align the `label2id` mapping between a `datasets.Dataset` and a classifier (e.g. a transformer with a `PretrainedConfig.label2id` dict), with the alignment performed on the dataset itself.

This will help us with the Hub evaluation, where we won't know in advance whether a model that is fine-tuned on say MNLI has the same mappings as the MNLI dataset we load from `datasets`.

An example where this is needed is if we naively try to evaluate `microsoft/deberta-base-mnli` on `mnli` because the model config has the following mappings:

```python
  "id2label": {
    "0": "CONTRADICTION",
    "1": "NEUTRAL",
    "2": "ENTAILMENT"
  },
  "label2id": {
    "CONTRADICTION": 0,
    "ENTAILMENT": 2,
    "NEUTRAL": 1
  }
```

while the `mnli` dataset has the `contradiction` and `neutral` labels swapped:

```python
id2label = {0: 'entailment', 1: 'neutral', 2: 'contradiction'}
label2id = {'contradiction': 2, 'entailment': 0, 'neutral': 1}
```

As a result, we get a much lower accuracy during evaluation:

```python
from datasets import load_dataset
from transformers.trainer_utils import EvalPrediction
from transformers import AutoModelForSequenceClassification, Trainer

# load dataset for evaluation
mnli = load_dataset("glue", "mnli", split="test")
# load model
model_ckpt = "microsoft/deberta-base-mnli"
model = AutoModelForSequenceClassification.from_pretrained(checkpoint)
# preprocess, create trainer ...
mnli_enc = ...
trainer = Trainer(model, args=args, tokenizer=tokenizer)
# generate preds
preds = trainer.predict(mnli_enc)
# preds.label_ids misalinged with model.config => returns wrong accuracy (too low)!
compute_metrics(EvalPrediction(preds.predictions, preds.label_ids))
```

The fix is to use the helper function before running the evaluation to make sure the label IDs are aligned:

```python
mnli_enc_aligned = mnli_enc.align_labels_with_mapping(label2id=config.label2id, label_column="label")
# preds now aligned and everyone is happy :)
preds = trainer.predict(mnli_enc_aligned)
```

cc @thomwolf @lhoestq 
@lhoestq i think this is ready for another review 🙂  @lhoestq thanks for the feedback - it's now integrated :) 

i also added a comment about sorting the input label IDs Created the PR here: https://github.com/huggingface/datasets/pull/2510 > Thanks ! Looks all good now :)
> 
> We will also need to have the `DatasetDict.align_labels_with_mapping` method. Let me quickly add it

thanks a lot! i always forget about `DatasetDict` - will be happy when it's just one "dataset" object :) So, there seems to be a problem with the function align_labels_with_mapping for models like this: https://huggingface.co/huggingface/distilbert-base-uncased-finetuned-mnli]. At least with this model, but perhaps also with others, the model.config.label2id values are of type str not int, which crashes said function. After manually converting the model.config.label2id values to int, the script runs smoothly.

