Add `min_new_tokens` argument in generate() (implementation based on `MinNewTokensLengthLogitsProcessor`)
Author: silverriver
Pull request: yes

# What does this PR do?
Fixes #20756 #20814 #20614 (cc @gonced8 @kotikkonstantin)

As many said, it is better to add an argument `min_new_tokens` to the `.generate()` method to limit the length of newly generated tokens. The current parameter `min_length` limits the length of `prompt + newly generated tokens`, not the length of `newly generated tokens`.

I closed my old PR #20819 and implement this feature based on `MinNewTokensLengthLogitsProcessor` (see #20892) as suggested by @gante .

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Comments:
_The documentation is not available anymore as the PR was closed or merged._

(@sgugger ready to merge if you agree. For context: this PR makes the `MinNewTokensLengthLogitsProcessor` usable from `.generate`, if the user passes `min_new_tokens` in the generate config or as an argument)