Adding new argument `max_new_tokens` for generate.
Author: Narsil
Pull request: yes

# What does this PR do?

This is a proposal to add a new argument `max_new_tokens` to `generate`.
This include a `MaxNewTokensCriteria` that enables callers that don't
know about the token length ahead (like pipelines callers) to manage
more easily the length of their generated output.

`max_length` is a hard to use argument for generate:

- It means different things in `encoder-decoder` context and
`decoder-only` context
  - `encoder-decoder`: max_length = max_new_tokens - 1 (in case of bos)
  - `decoder-only`: max_length = input_ids.shape[-1] + max_new_tokens
- It is hard to understand from a pipeline point of view where `tokens`
do not exist yet.


This is a proposal to add a new argument `max_new_tokens` to `generate`.
This include a `MaxNewTokensCriteria` which is a bit redundant with
respect to `MaxLengthCriteria`. It is a consistency concern for now but debattable.



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Fixes # (issue)


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## Who can review?

@LysandreJik  @patrickvonplaten 

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Comments:
@LysandreJik @patrickvonplaten

Forgot to add you for review, my bad.