Exclude the madeup words from M2M100Tokenizer.vocab_size
# What does this PR do?

The `<unk>` token has an incorrect ID in `M2M100Tokenizer.get_vocab`:

```python
>>> tokenizer = transformers.M2M100Tokenizer.from_pretrained("facebook/m2m100_418M")
>>> tokenizer.convert_tokens_to_ids("<unk>")
3
>>> tokenizer.get_vocab()["<unk>"]
128111
```

The reason is the vocabulary is defined like this:

```
vocab = {self.convert_ids_to_tokens(i): i for i in range(self.vocab_size)}
```

but `_convert_id_to_token` converts the "madeup words" to `<unk>`.

We can fix this issue by excluding the "madeup words" from the vocabulary size, which is consistent with how other tokenizers work such as `NllbTokenizer`.

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- [x] Did you write any new necessary tests?


## Who can review?

@ArthurZucker
_The documentation is not available anymore as the PR was closed or merged._<|||||>Hi @ArthurZucker, do you have some time to review this PR? Thanks!