Fixed docs for the shape of `scores` in `generate()`
Author: kylie-box
Pull request: yes

# What does this PR do?
Fixed the shape of `scores` from `generate()` outputs to be `max_length-1` for all output classes. The first token `decoder_start_token_id` is not generated and thus no scores. 

[Scores in generate()](https://discuss.huggingface.co/t/scores-in-generate/3450)
[Generation Probabilities: How to compute probabilities of output scores for GPT2](https://discuss.huggingface.co/t/generation-probabilities-how-to-compute-probabilities-of-output-scores-for-gpt2/3175)

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Comments:
This issue has been automatically marked as stale because it has not had recent activity. If you think this still needs to be addressed please comment on this thread.

Please note that issues that do not follow the [contributing guidelines](https://github.com/huggingface/transformers/blob/master/CONTRIBUTING.md) are likely to be ignored.

Hey @kylie-box, 

Could you run `make style` to fix a problem with the code quality. I think we can merge afterward :-)

Hey @patrickvonplaten,

It's fixed now. :)  


Thanks a lot!