Fix generation min length
Author: patrickvonplaten
Pull request: yes

# What does this PR do?

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This PR fixes a typo in `generation_utils.py`. `min_length` of `0` has no effect so we don't need to check whether it's bigger than -1, but whether it's bigger than 0.


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Models:

- albert, bert, xlm: @LysandreJik
- blenderbot, bart, marian, pegasus, encoderdecoder,  t5: @patrickvonplaten, @patil-suraj
- longformer, reformer, transfoxl, xlnet: @patrickvonplaten
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Library:

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Comments:
The docs for this PR live [here](https://moon-ci-docs.huggingface.co/docs/transformers/pr_16206). All of your documentation changes will be reflected on that endpoint.

> LGTM.
> 
> If I understand correctly we were almost always adding the min_length logits processor even though it's not needed when min length is zero right ? (curiosity Can we generate zero-tokens actually ?)

Yeah that was wrong before - we shouldn't use the min_length logits processor if min_length=0