[TokenClassification] Label realignment for subword aggregation
# What does this PR do?

Fixes #10263, #10763
See also #10568

## Before submitting
- [ ] This PR fixes a typo or improves the docs (you can dismiss the other checks if that's the case).
- [x] Did you read the [contributor guideline](https://github.com/huggingface/transformers/blob/master/CONTRIBUTING.md#start-contributing-pull-requests),
      Pull Request section?
- [x] Was this discussed/approved via a Github issue or the [forum](https://discuss.huggingface.co/)? Please add a link
      to it if that's the case.
- [ ] Did you make sure to update the documentation with your changes? Here are the
      [documentation guidelines](https://github.com/huggingface/transformers/tree/master/docs), and
      [here are tips on formatting docstrings](https://github.com/huggingface/transformers/tree/master/docs#writing-source-documentation).
- [x] Did you write any new necessary tests?


## Who can review?
@LysandreJik
@sgugger 

## What capabilities have been added ?
label realignment: token predictions for subwords can be realigned with 4 different strategies

`first` (default): the prediction for the first token in the word is assigned to all subword tokens
`max`: the highest confidence prediction among the subword tokens is assigned to all subword tokens
`average`: the average pool of the predictions for all subwords is assigned to all subword tokens

## What are the expected changes from the current behavior?
New flag `aggregation_strategy` enables realignment.
Already existing flag `ignore_subwords` actually enables merging subwords.

## Example use cases with code sample enabled by the PR
```
ner = transformers.pipeline(
    'ner',
    model='elastic/distilbert-base-cased-finetuned-conll03-english',
    tokenizer='elastic/distilbert-base-cased-finetuned-conll03-english',
    ignore_labels=[],
    ignore_subwords=False,
    aggregation_strategy='average'
)
ner('Mark Musterman')
[
    {
        'word': 'Mark',
        'score': 0.999686598777771,
        'index': 1,
        'start': 0,
        'end': 4,
        'is_subword': False,
        'entity': 'B-PER'
    },
    {
        'word': 'Must',
        'score': 0.9995412826538086,
        'index': 2,
        'start': 5,
        'end': 9,
        'is_subword': False,
        'entity': 'I-PER'
    },
    {
        'word': '##erman',
        'score': 0.9996127486228943,
        'index': 3,
        'start': 9,
        'end': 14,
        'is_subword': True,
        'entity': 'I-PER'
    }
]
ner = transformers.pipeline(
    'ner',
    model='elastic/distilbert-base-cased-finetuned-conll03-english',
    tokenizer='elastic/distilbert-base-cased-finetuned-conll03-english',
    ignore_labels=[],
    ignore_subwords=True,
    aggregation_strategy='average'
)
ner('Mark Musterman')
[
    {
        'word': 'Mark',
        'score': 0.999686598777771,
        'index': 1,
        'start': 0,
        'end': 4,
        'is_subword': False,
        'entity': 'B-PER'
    },
    {
        'word': 'Musterman',
        'score': 0.9995412826538086,
        'index': 2,
        'start': 5,
        'end': 9,
        'is_subword': False,
        'entity': 'I-PER'
    }
]
```

## Previous use cases with code sample that see the behavior changes
```
ner = transformers.pipeline(
    'ner',
    model='elastic/distilbert-base-cased-finetuned-conll03-english',
    tokenizer='elastic/distilbert-base-cased-finetuned-conll03-english',
    ignore_labels=[],
    ignore_subwords=True
)
ner('Mark Musterman')
[
    {
        'word': 'Mark',
        'score': 0.999686598777771,
        'entity': 'B-PER',
        'index': 1,
        'start': 0,
        'end': 4
    },
    {
        'word': 'Must',
        'score': 0.9995412826538086,
        'entity': 'I-PER',
        'index': 2,
        'start': 5,
        'end': 9
    },
    {
        'word': '##erman',
        'score': 0.9996127486228943,
        'entity': 'I-PER',
        'index': 3,
        'start': 9,
        'end': 14
    }
]
```
@sgugger the failed test (`test_gpt2_model_past_large_inputs`) seems unrelated to the changes in this PR. Any thought on what might be going on and how to resolve?<|||||>No, it's just flaky, don't worry!<|||||>Hey @francescorubbo, @Narsil pointed out a few issues with the current implementation that we'll take a look at today/tomorrow. Namely, the code becomes a bit complex as we keep adding features to that pipeline so it might be time for a slightly larger refactor, and some code is model-specific, such as this line which wouldn't work on non BERT-like tokenizers:

```
subwords[0]["word"] += "".join([sub["word"].split("##")[1] for sub in subwords[1:]])
```

We're taking a look at what can be done and will come back to you in a bit. Thanks again for your patience.<|||||>This issue has been automatically marked as stale because it has not had recent activity. If you think this still needs to be addressed please comment on this thread.

Please note that issues that do not follow the [contributing guidelines](https://github.com/huggingface/transformers/blob/master/CONTRIBUTING.md) are likely to be ignored.