Diverse beam search 2
Author: patrickvonplaten
Pull request: yes

# What does this PR do?

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Copy of #8627 because branch got messed up.


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 albert, bert, XLM: @LysandreJik
 GPT2: @LysandreJik, @patrickvonplaten
 tokenizers: @mfuntowicz
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 Benchmarks: @patrickvonplaten
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 nlp datasets: [different repo](https://github.com/huggingface/nlp)
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Comments:
@ayushtiku5 I tweeted about it and tagged you on the tweet: https://twitter.com/PatrickPlaten/status/1336681238485229568 - hope that's fine for you :-) 

@ayushtiku5 can you please check if `HammingDiversityLogitsProcessor` and `PrefixConstrainedLogitsProcessor` can be speeded up with functions like `torch.scatter`, `torch.gather`, `torch.masked_fill`, `torch.index_fill`, `torch.index_add`, `torch.index_copy`? I believe there is room for improvement in 
https://github.com/huggingface/transformers/blob/master/src/transformers/generation_logits_process.py#L406-L409 and https://github.com/huggingface/transformers/blob/master/src/transformers/generation_logits_process.py#L471 (in the way like https://github.com/huggingface/transformers/pull/9557 and https://github.com/huggingface/transformers/pull/9600) but I have no experience using these processors to create good enough examples for speed testing and corner cases research.