Add FlaxMinNewTokensLengthLogitsProcessor
Author: yeandy
Pull request: yes

# What does this PR do?

PyTorch's options of logit processors has both `MinLengthLogitsProcessor` and `MinNewTokensLengthLogitsProcessor`. See [code](https://github.com/huggingface/transformers/blob/v4.30.2/src/transformers/generation/utils.py#L886C1-L901C14).

However, Flax only has [FlaxMinLengthLogitsProcessor](https://github.com/huggingface/transformers/blob/v4.30.2/src/transformers/generation/flax_utils.py#L500C1-L507C14), and does not account for the case if `min_token_length` is passed into the GenerationConfig (which is what PyTorch's `MinNewTokensLengthLogitsProcessor` does. As a result, when passing the same config that contains `min_new_tokens` to both the PyTorch and Flax model, I see different generated outputs.

Changes:
- Add `FlaxMinTokensLengthLogitsProcessor` class, which is the Flax version of PyTorch's `MinNewTokensLengthLogitsProcessor`, and add an if-statement to select `FlaxMinTokensLengthLogitsProcessor` when `min_new_tokens` is passed.
- Change the conditional statement for `FlaxMinLengthLogitsProcessor`. I believe it's a bug, where it checks for `generation_config.min_length > -1`. However, the [default value is 0](https://github.com/huggingface/transformers/blob/v4.30.2/src/transformers/generation/configuration_utils.py#L228), so this expression will always be true, and won't allow us to go the if statement for `FlaxMinTokensLengthLogitsProcessor`. Checking that `generation_config.min_length > 0` is also how the PyTorch logic [does it](https://github.com/huggingface/transformers/blob/v4.30.2/src/transformers/generation/utils.py#L889).


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