Add the Perplexity metric

[bryant1410]

What does this PR do?

Fixes #<issue_number>

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• Did you write any new necessary tests?

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[codecov]

Codecov Report

Merging #922 (c0e21aa) into master (b49e579) will decrease coverage by 44%.
The diff coverage is 90%.

@@           Coverage Diff           @@
##           master   #922     +/-   ##
=======================================
- Coverage      94%    50%    -44%     
=======================================
  Files         180    182      +2     
  Lines        8082   8141     +59     
=======================================
- Hits         7596   4059   -3537     
- Misses        486   4082   +3596     

[Borda]

need requires testing 🐰

[stancld]

@bryant1410 Thank you a lot for contributing! 🎉

I have one issue regarding the API of this new metric so that it will better correspond to the design of torchmetrics.

[bryant1410]

need requires testing rabbit

@Borda what do you mean? To add more tests? Specifically, testing what behavior?

[stancld]

@Borda Looks like InfoLM is now really ready for review, then I hope I'll prepare this one tomorrow O:]