title: Fix/start token mask issue and update documentation
comments: _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.
body: 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