Removed `max_length` from being mandatory within `generate`.
Author: Narsil
Pull request: yes

# What does this PR do?

- Moving on to fully using `StoppingCriteria` for `greedy` and `sample`
modes.
- `max_length` still used for `beam_search` and `group_beam_search`
(Follow up PR)
- Fixes a bug with MaxLengthStoppingCriteria (we should stop as soon a
we hit the max_length, the comparison needs to be or equal, that affects
the tests).
- Added options to use `logits_processor` and `stopping_criteria`
directly within `generate` function (so some users can define their own
`logits_processor` and `stopping_criteria`).
- Modified the backward compat tests to make sure we issue a warning.


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Fixes # (issue)


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## Who can review?

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@patrickvonplaten @LysandreJik

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Comments:
All tests pass, the max_length was actually a bug hidden by  `while cur_len < max_length` that was still in there.
The bart tests (at least) caught it automatically and enabled me to change it to the correct comparison ! 

> All tests pass, the max_length was actually a bug hidden by `while cur_len < max_length` that was still in there.
> The bart tests (at least) caught it automatically and enabled me to change it to the correct comparison !

Great! Feel free to merge then!