Fix/start token mask issue and update documentation
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_The documentation is not available anymore as the PR was closed or merged._ > Good catch ! Thanks :)
> 
> Next time can you describe your fix in the Pull Request description please ?

Thanks. Also whoops, sorry about not being very descriptive. I updated the pull request description, and will keep this in mind for future PRs.
This pr fixes a couple bugs:

1) the perplexity was calculated with a 0 in the attention mask for the start token, which was causing high perplexity scores that were not correct
2) the documentation was not updated
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