inject automatic end of utterance tokens
This adds a new feature:

For select models add `<end_of_utterance>` token at the end of each utterance.

The user can now easily break up their prompt and not need to worry about messing with tokens.

So for this prompt:

```
    [
        "User:",
        image,
        "Describe this image.",
        "Assistant: An image of two kittens in grass.",
        "User:",
        "https://hips.hearstapps.com/hmg-prod/images/dog-puns-1581708208.jpg",
        "Describe this image.",
        "Assistant:",
    ],
```

this new code with add_end_of_utterance_token=True will generate:

`full_text='<s>User:<fake_token_around_image><image><fake_token_around_image>Describe this image.<end_of_utterance>Assistant: An image of two kittens in grass.<end_of_utterance>User:<fake_token_around_image><image><fake_token_around_image>Describe this image.<end_of_utterance>Assistant:'
`

_The documentation is not available anymore as the PR was closed or merged._<|||||>The docs for this PR live [here](https://moon-ci-docs.huggingface.co/docs/transformers/pr_25218). All of your documentation changes will be reflected on that endpoint.